Add av1 to MediaSource.isTypeSupported

When av1 video playback is enabled, declare it as supported in the webm container in MediaSource.IsTypeSupported.

Also support special mime types of the form video/webm; codecs=vp9.experimental.<git-commit-id> so test sites can verify playback support of particular encodings while the av1 bitstream is under development.

dom/media/webm/WebMDecoder.cpp

@@ -5,6 +5,9 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/Preferences.h"
+#ifdef MOZ_AV1
+#include "AOMDecoder.h"
+#endif
 #include "MediaDecoderStateMachine.h"
 #include "WebMDemuxer.h"
 #include "WebMDecoder.h"
@@ -65,6 +68,11 @@ WebMDecoder::CanHandleMediaType(const nsACString& aMIMETypeExcludingCodecs,
 
       continue;
     }
+#ifdef MOZ_AV1
+    if (isVideo && AOMDecoder::IsSupportedCodec(codec)) {
+      continue;
+    }
+#endif
     // Some unsupported codec.
     return false;
   }