Bug 1065827 - Do not always wait to decode frames to emit loadedmetadata. r=jwwang

dom/html/HTMLMediaElement.cpp

@@ -2903,11 +2903,11 @@ void HTMLMediaElement::MetadataLoaded(const MediaInfo* aInfo,
   // If this element had a video track, but consists only of an audio track now,
   // delete the VideoFrameContainer. This happens when the src is changed to an
   // audio only file.
-  if (!aInfo->HasVideo() && mVideoFrameContainer) {
-    // call ForgetElement() such that callbacks from |mVideoFrameContainer|
-    // won't reach us anymore.
-    mVideoFrameContainer->ForgetElement();
-    mVideoFrameContainer = nullptr;
+  // Else update its dimensions.
+  if (!aInfo->HasVideo()) {
+    ResetState();
+  } else {
+    UpdateMediaSize(aInfo->mVideo.mDisplay);
   }
 }