webcodecs: Don't readback frames if they have GPU buffers

Currently VideoEncoder will readback frames as RGB even if they have
mappable NV12 GPU buffers. There is no need to do it underlying
encoders can map GPU buffers just fine.

Test: https://wcwc.glitch.me/
Bug: 1166962
Change-Id: Ie72c65c4df114264a70b9da337af27a934a8b74a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631407Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843966}

third_party/blink/renderer/modules/webcodecs/video_encoder.cc

@@ -456,7 +456,7 @@ void VideoEncoder::ProcessEncode(Request* request) {
 
   scoped_refptr<media::VideoFrame> frame = request->frame->frame();
 
-  if (frame->HasTextures()) {
+  if (frame->HasTextures() && !frame->HasGpuMemoryBuffer()) {
     frame = ReadbackTextureBackedFrameToMemory(std::move(frame));
     if (!frame) {
       auto status = media::Status(media::StatusCode::kEncoderFailedEncode,