Implement modulepreload in early hints

The aEarlyHintPreloaderId parameter for StartLoad/StartLoadInternal is changed
to be a member variable of ScriptLoadRequest instead so that an initiator type
of early hints can be set for module requests. Before, ModuleLoader would always
pass in a zero value for the id since ModuleLoaderBase has no concept of early
hints when it calls StartFetch.

As a prerequisite for early hints support, this commit also implements
modulepreload in link headers (Bug 1773056).

Differential Revision: https://phabricator.services.mozilla.com/D180020

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1798319
gecko-commit: 5cafcb0a03c862dea23b88faae77b0e79693fcc6
gecko-reviewers: manuel, smaug, necko-reviewers, kershaw

loading/early-hints/modulepreload-as-worker-cross-origin.h2.window.js

@@ -0,0 +1,15 @@
+// META: script=/common/utils.js
+// META: script=resources/early-hints-helpers.sub.js
+
+// see modulepreload-in-early-hints.h2.window.js for params explanation
+test(() => {
+    const params = new URLSearchParams();
+    params.set("description",
+        'Modulepreload should not load with as="worker" from cross-origin url');
+    params.set("resource-url",
+        CROSS_ORIGIN_RESOURCES_URL + "/empty.js?" + token());
+    params.set("as", "worker");
+    params.set("should-preload", false);
+    const test_url = "resources/modulepreload-in-early-hints.h2.py?" + params.toString();
+    window.location.replace(new URL(test_url, window.location));
+});

loading/early-hints/modulepreload-as-worker.h2.window.js

@@ -0,0 +1,15 @@
+// META: script=/common/utils.js
+// META: script=resources/early-hints-helpers.sub.js
+
+// see modulepreload-in-early-hints.h2.window.js for params explanation
+test(() => {
+    const params = new URLSearchParams();
+    params.set("description",
+        'Modulepreload should load with as="worker" from same-origin url');
+    params.set("resource-url",
+        SAME_ORIGIN_RESOURCES_URL + "/empty.js?" + token());
+    params.set("as", "worker");
+    params.set("should-preload", true);
+    const test_url = "resources/modulepreload-in-early-hints.h2.py?" + params.toString();
+    window.location.replace(new URL(test_url, window.location));
+});

loading/early-hints/modulepreload-cross-origin.h2.window.js

@@ -0,0 +1,13 @@
+// META: script=/common/utils.js
+// META: script=resources/early-hints-helpers.sub.js
+
+// see modulepreload-in-early-hints.h2.window.js for params explanation
+test(() => {
+    const params = new URLSearchParams();
+    params.set("description", "Modulepreload works in early hints from cross-origin url");
+    params.set("resource-url",
+        CROSS_ORIGIN_RESOURCES_URL + "/empty.js?" + token());
+    params.set("should-preload", true);
+    const test_url = "resources/modulepreload-in-early-hints.h2.py?" + params.toString();
+    window.location.replace(new URL(test_url, window.location));
+});

loading/early-hints/modulepreload-in-early-hints.h2.window.js

@@ -1,10 +1,20 @@
 // META: script=/common/utils.js
 // META: script=resources/early-hints-helpers.sub.js
 
+// params are sent to a Python handler[1] that returns a 103 Early Hints
+// response based the values of "resource-url" and "as", and then that response
+// is validated by a window test[2] according to the value of "should-preload"
+//
+// see: https://web-platform-tests.org/writing-tests/h2tests.html
+//
+// [1]: resources/modulepreload-in-early-hints.h2.py
+// [2]: resources/modulepreload-in-early-hints.h2.html
 test(() => {
     const params = new URLSearchParams();
+    params.set("description", "Modulepreload works in early hints");
     params.set("resource-url",
         SAME_ORIGIN_RESOURCES_URL + "/empty.js?" + token());
+    params.set("should-preload", true);
     const test_url = "resources/modulepreload-in-early-hints.h2.py?" + params.toString();
     window.location.replace(new URL(test_url, window.location));
 });

loading/early-hints/resources/modulepreload-in-early-hints.h2.py

@@ -3,7 +3,12 @@
 
 def handle_headers(frame, request, response):
     resource_url = request.GET.first(b"resource-url").decode()
-    link_header_value = "<{}>; rel=modulepreload".format(resource_url)
+    as_value = request.GET.first(b"as", None)
+    if as_value:
+        link_header_value = "<{}>; rel=modulepreload; as={}".format(
+            resource_url, as_value.decode())
+    else:
+        link_header_value = "<{}>; rel=modulepreload".format(resource_url)
     early_hints = [
         (b":status", b"103"),
         (b"link", link_header_value),

loading/early-hints/resources/modulepreload-in-early-hints.html

@@ -16,11 +16,14 @@
     });
 }
 
+const params = new URLSearchParams(window.location.search);
+const description = params.get("description");
+
 promise_test(async (t) => {
-    const params = new URLSearchParams(window.location.search);
     const resource_url = params.get("resource-url");
+    const should_preload = params.get("should-preload") === "true";
     await fetchModuleScript(resource_url);
-    assert_true(isPreloadedByEarlyHints(resource_url));
-}, "Modulepreload in an early hints.");
+    assert_equals(isPreloadedByEarlyHints(resource_url), should_preload);
+}, description);
 </script>
 </body>