Bug 1870346 - Add WebCodecs pref to force decoding of H264 on OSX for testing. r=media-playback-reviewers,chunmin

This is needed so that we can write talos tests that exercise different
codecs on Windows/OSX/Linux. We can't distinguish between platforms to
skip or gracefully fail this test for a particular platform, and since
the H264 we are testing with is able to be decoded successfully, let's
just make a pref to allow us (and others) to test with what we have
implemented even if partially broken.

Differential Revision: https://phabricator.services.mozilla.com/D196603

dom/media/webcodecs/VideoDecoder.cpp

@@ -19,6 +19,7 @@
 #include "mozilla/DebugOnly.h"
 #include "mozilla/Logging.h"
 #include "mozilla/Maybe.h"
+#include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/Try.h"
 #include "mozilla/Unused.h"
 #include "mozilla/dom/EncodedVideoChunk.h"
@@ -232,7 +233,8 @@ static bool CanDecode(const Config& aConfig) {
   if (!IsSupportedCodec(param.mParsedCodec)) {
     return false;
   }
-  if (IsOnMacOS() && IsH264CodecString(param.mParsedCodec)) {
+  if (IsOnMacOS() && IsH264CodecString(param.mParsedCodec) &&
+      !StaticPrefs::dom_media_webcodecs_force_osx_h264_enabled()) {
     // This will be fixed in Bug 1846796.
     return false;
   }

modules/libpref/init/StaticPrefList.yaml

@@ -3042,6 +3042,11 @@
   value: @IS_NIGHTLY_BUILD@
   mirror: always
 
+- name: dom.media.webcodecs.force-osx-h264-enabled
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 # Number of seconds of very quiet or silent audio before considering the audio
 # inaudible.
 - name: dom.media.silence_duration_for_audibility