Bug 1688879 - Part 1: Add Pref for import maps. r=jonco,yulia

Add a pref "dom.importMaps.enabled", with default false.

Differential Revision: https://phabricator.services.mozilla.com/D142069

dom/base/Document.cpp

@@ -17539,6 +17539,11 @@ bool Document::ModuleScriptsEnabled() {
          StaticPrefs::dom_moduleScripts_enabled();
 }
 
+bool Document::ImportMapsEnabled() {
+  return nsContentUtils::IsChromeDoc(this) ||
+         StaticPrefs::dom_importMaps_enabled();
+}
+
 void Document::ReportShadowDOMUsage() {
   nsPIDOMWindowInner* inner = GetInnerWindow();
   if (NS_WARN_IF(!inner)) {

dom/base/Document.h

@@ -3986,6 +3986,8 @@ class Document : public nsINode,
 
   bool ModuleScriptsEnabled();
 
+  bool ImportMapsEnabled();
+
   /**
    * Find the (non-anonymous) content in this document for aFrame. It will
    * be aFrame's content node if that content is in this document and not

modules/libpref/init/StaticPrefList.yaml

@@ -2797,6 +2797,12 @@
   value: true
   mirror: always
 
+# Is support for import maps (<script type="importmap">) enabled for content?
+- name: dom.importMaps.enabled
+  type: bool
+  value: false
+  mirror: always
+
 # Whether we disable triggering mutation events for changes to style
 # attribute via CSSOM.
 # NOTE: This preference is used in unit tests. If it is removed or its default