Handle Alpha channel in Canvas capture

chrome/renderer/media/cast_rtp_stream.cc

Index: chrome/renderer/media/cast_rtp_stream.cc
diff --git a/chrome/renderer/media/cast_rtp_stream.cc b/chrome/renderer/media/cast_rtp_stream.cc
index 755cfd372b4f37402b3c3d41be5551e1667c6c50..f5dac133b174d666591f61363f5620984b874006 100644
--- a/chrome/renderer/media/cast_rtp_stream.cc
+++ b/chrome/renderer/media/cast_rtp_stream.cc
@@ -329,6 +329,18 @@ class CastVideoSink : public base::SupportsWeakPtr<CastVideoSink>,
                                           ? base::TimeTicks::Now()
                                           : estimated_capture_time;
 
+    if (!(frame->IsMappable() &&
+          (frame->format() == media::PIXEL_FORMAT_I420 ||
+           frame->format() == media::PIXEL_FORMAT_YV12 ||
+           frame->format() == media::PIXEL_FORMAT_YV12A))) {
+      NOTREACHED();
+      return;
+    }
+
+    // Drop alpha channel since we do not support it yet.
+    if (frame->format() == media::PIXEL_FORMAT_YV12A)
+      frame->DropYV12AAlphaChannel();
+
     // Used by chrome/browser/extension/api/cast_streaming/performance_test.cc
     TRACE_EVENT_INSTANT2(
         "cast_perf_test", "MediaStreamVideoSink::OnVideoFrame",