Add VEAEncoder to VideoTrackRecorder

This CL introduces VEAEncoder which is responsible for handling
GPU encoder interactions for VideoTrackRecorder. VideoTrackRecorder
prioritizes VEAEncoders before SW implementations.

This CL is [2 of 2] to add support for Video Encode Accelerator in
VideoTrackRecorder.
Playground CL: https://codereview.chromium.org/1934093002/

BUG=608385
TEST=https://cdn.rawgit.com/miguelao/demos/master/mediarecorder.html

Review-Url: https://codereview.chromium.org/1990643002
Cr-Commit-Position: refs/heads/master@{#395464}

content/renderer/media/media_recorder_handler.cc

@@ -40,10 +40,8 @@ media::VideoCodec CodecIdToMediaVideoCodec(VideoTrackRecorder::CodecId id) {
       return media::kCodecVP8;
     case VideoTrackRecorder::CodecId::VP9:
       return media::kCodecVP8;
-#if BUILDFLAG(RTC_USE_H264)
     case VideoTrackRecorder::CodecId::H264:
       return media::kCodecH264;
-#endif
   }
   NOTREACHED() << "Unsupported codec";
   return media::kUnknownVideoCodec;