Bug 1629529 - fix shutdown dispatch assert in WebrtcTCPSocket. r=bwc

If we're shutting down and mSocketThread is no longer accepting work,
this dispatch can fail.  As far as I can see, fully fixing this would
require changes to nsIThread in order to allow checking whether the
thread is shutting down.

Differential Revision: https://phabricator.services.mozilla.com/D71964

media/mtransport/ipc/WebrtcTCPSocket.cpp

@@ -106,9 +106,17 @@ void WebrtcTCPSocket::CloseWithReason(nsresult aReason) {
     // Let's pretend we got an open even if we didn't to prevent an Open later.
     mOpened = true;
 
-    MOZ_ALWAYS_SUCCEEDS(mSocketThread->Dispatch(NewRunnableMethod<nsresult>(
-        "WebrtcTCPSocket::CloseWithReason", this,
-        &WebrtcTCPSocket::CloseWithReason, aReason)));
+    DebugOnly<nsresult> rv =
+        mSocketThread->Dispatch(NewRunnableMethod<nsresult>(
+            "WebrtcTCPSocket::CloseWithReason", this,
+            &WebrtcTCPSocket::CloseWithReason, aReason));
+
+    // This was MOZ_ALWAYS_SUCCEEDS, but that now uses MOZ_DIAGNOSTIC_ASSERT.
+    // In order to convert this back to MOZ_ALWAYS_SUCCEEDS we would need
+    // OnSocketThread to return true if we're shutting down and doing the
+    // "running all of STS's queued events on main" thing.
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
     return;
   }