Bug 1685399 - part13 : add test. r=padenot

As we can only snapshot a telemetry histogram in the chrome process, we have to make them measurable in the chrome process and write a chrome mochitest.

Differential Revision: https://phabricator.services.mozilla.com/D101266

dom/html/HTMLMediaElement.cpp

@@ -7137,6 +7137,13 @@ FrameStatistics* HTMLMediaElement::GetFrameStatistics() const {
   return mDecoder ? &(mDecoder->GetFrameStatistics()) : nullptr;
 }
 
+void HTMLMediaElement::DispatchAsyncTestingEvent(const nsAString& aName) {
+  if (!StaticPrefs::media_testing_only_events()) {
+    return;
+  }
+  DispatchAsyncEvent(aName);
+}
+
 void HTMLMediaElement::AudioCaptureTrackChange(bool aCapture) {
   // No need to capture a silent media element.
   if (!HasAudio()) {