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/platforms/agnostic/AOMDecoder.cpp

@@ -210,6 +210,19 @@ AOMDecoder::IsAV1(const nsACString& aMimeType)
          || aMimeType.EqualsLiteral("video/av1");
 }
 
+/* static */
+bool
+AOMDecoder::IsSupportedCodec(const nsAString& aCodecType)
+{
+  // While AV1 is under development, we describe support
+  // for a specific aom commit hash so sites can check
+  // compatibility.
+  auto version = NS_ConvertASCIItoUTF16("av1.experimental.");
+  version.AppendLiteral("4d668d7feb1f8abd809d1bca0418570a7f142a36");
+  return aCodecType.EqualsLiteral("av1") ||
+         aCodecType.Equals(version);
+}
+
 /* static */
 bool
 AOMDecoder::IsKeyframe(Span<const uint8_t> aBuffer) {

dom/media/platforms/agnostic/AOMDecoder.h

@@ -34,6 +34,9 @@ public:
   // by our demuxers to identify AV1 streams.
   static bool IsAV1(const nsACString& aMimeType);
 
+  // Return true if aCodecType is a supported codec description.
+  static bool IsSupportedCodec(const nsAString& aCodecType);
+
   // Return true if a sample is a keyframe.
   static bool IsKeyframe(Span<const uint8_t> aBuffer);
 

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;
   }