Allow planar access for I420A VideoFrames.

These appear to be commonly generated from the canvas capture path
when '{alpha: false}' is not specified. They may also be generated
from alpha vp8/vp9 decoding

R=sandersd

Fixed: 1148800
Change-Id: I5da2aed405b6ed7c0708dae1aee2bb397731c26e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542936Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828931}

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

@@ -219,10 +219,12 @@ VideoFrame* VideoFrame::Create(ImageBitmap* source,
 
 // static
 bool VideoFrame::IsSupportedPlanarFormat(media::VideoFrame* frame) {
-  // For now only I420 or NV12 in CPU or GPU memory is supported.
+  // For now only I420, I420A, or NV12 in CPU or GPU memory is supported.
   return frame && (frame->IsMappable() || frame->HasGpuMemoryBuffer()) &&
          ((frame->format() == media::PIXEL_FORMAT_I420 &&
            frame->layout().num_planes() == 3) ||
+          (frame->format() == media::PIXEL_FORMAT_I420A &&
+           frame->layout().num_planes() == 4) ||
           (frame->format() == media::PIXEL_FORMAT_NV12 &&
            frame->layout().num_planes() == 2));
 }
@@ -234,6 +236,7 @@ String VideoFrame::format() const {
 
   switch (local_frame->format()) {
     case media::PIXEL_FORMAT_I420:
+    case media::PIXEL_FORMAT_I420A:
       return V8VideoPixelFormat(V8VideoPixelFormat::Enum::kI420);
     case media::PIXEL_FORMAT_NV12:
       return V8VideoPixelFormat(V8VideoPixelFormat::Enum::kNV12);

third_party/blink/renderer/modules/webcodecs/video_pixel_format.idl

@@ -5,7 +5,7 @@
 // https://github.com/WICG/web-codecs
 
 enum VideoPixelFormat {
-    // 4:2:0 subsampled 3 plane Y,U,V
+    // 4:2:0 subsampled 3 plane Y,U,V or 4 plane Y,U,V,A
     "I420",
 
     // 4:2:0 subsampled 2 plane Y,UV