Bug 1477011 - Report vp9 in mp4 as supported. r=dminor

Since we switched to the mp4 rust demuxer, VP9 in mp4 is always supported.

Differential Revision: https://phabricator.services.mozilla.com/D2261

dom/html/HTMLMediaElement.cpp

@@ -4778,16 +4778,6 @@ HTMLMediaElement::UnbindFromTree(bool aDeep, bool aNullParent)
   RunInStableState(task);
 }
 
-static bool
-IsVP9InMP4(const MediaContainerType& aContainerType)
-{
-  const MediaContainerType mimeType(aContainerType.Type());
-  return DecoderTraits::IsMP4SupportedType(
-           mimeType,
-           /* DecoderDoctorDiagnostics* */ nullptr) &&
-         IsVP9CodecString(aContainerType.ExtendedType().Codecs().AsString());
-}
-
 /* static */
 CanPlayStatus
 HTMLMediaElement::GetCanPlay(const nsAString& aType,
@@ -4799,14 +4789,6 @@ HTMLMediaElement::GetCanPlay(const nsAString& aType,
   }
   CanPlayStatus status =
     DecoderTraits::CanHandleContainerType(*containerType, aDiagnostics);
-  if (status == CANPLAY_YES && IsVP9InMP4(*containerType)) {
-    // We don't have a demuxer that can handle VP9 in non-fragmented MP4.
-    // So special-case VP9 in MP4 here, as we assume canPlayType() implies
-    // non-fragmented MP4 anyway. Note we report that we can play VP9
-    // in MP4 in MediaSource.isTypeSupported(), as the fragmented MP4
-    // demuxer can handle VP9 in fragmented MP4.
-    return CANPLAY_NO;
-  }
   if (status == CANPLAY_YES &&
       (*containerType).ExtendedType().Codecs().IsEmpty()) {
     // Per spec: 'Generally, a user agent should never return "probably" for a