Bug 1550633 - part2 : maybe start loading resource when the track's mode changes. r=jya

According to the spec [1], the user agent must start the track processing model when text track has its text track mode changed.

[1] https://html.spec.whatwg.org/multipage/media.html#sourcing-out-of-band-text-tracks:start-the-track-processing-model

Differential Revision: https://phabricator.services.mozilla.com/D31372

--HG--
extra : moz-landing-system : lando

dom/html/HTMLTrackElement.cpp

@@ -256,6 +256,11 @@ void HTMLTrackElement::MaybeDispatchLoadResource() {
     return;
   }
 
+  if (ReadyState() == TextTrackReadyState::Loaded) {
+    LOG(LogLevel::Info, ("%p Has already loaded resource", this));
+    return;
+  }
+
   // step5, await a stable state and run the rest of steps.
   if (!mLoadResourceDispatched) {
     RefPtr<WebVTTListener> listener = new WebVTTListener(this);

dom/media/TextTrack.cpp

@@ -129,6 +129,12 @@ void TextTrack::SetMode(TextTrackMode aValue) {
   if (mediaElement) {
     mediaElement->NotifyTextTrackModeChanged();
   }
+  // https://html.spec.whatwg.org/multipage/media.html#sourcing-out-of-band-text-tracks:start-the-track-processing-model
+  // Run the `start-the-track-processing-model` to track's corresponding track
+  // element whenever track's mode changes.
+  if (mTrackElement) {
+    mTrackElement->MaybeDispatchLoadResource();
+  }
   // Ensure the TimeMarchesOn is called in case that the mCueList
   // is empty.
   NotifyCueUpdated(nullptr);