Handle Alpha channel in Canvas capture

media/base/video_frame.h

Index: media/base/video_frame.h
diff --git a/media/base/video_frame.h b/media/base/video_frame.h
index ad7f4c01a4268651728ce71acba87c3ec0ac388e..ea1359e32ad3a6764bbbf73b81a81c7b30899f8c 100644
--- a/media/base/video_frame.h
+++ b/media/base/video_frame.h
@@ -379,6 +379,9 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
   CVPixelBufferRef cv_pixel_buffer() const;
 #endif
 
+  // Converts a frame with YV12A format into I420 by dropping alpha channel.

[Pawel Osciak, 2016/02/26 00:32:51]

Personally, I'm wondering if this kind of a method should be in VideoFrame
itself, not somewhere else? VF is quite complicated already...

[emircan, 2016/02/26 01:33:08]

I see your point about VF being too complicated again. However, I am modifying
private class members in this function and it belongs here more than a helper
class. WDYT?

[Pawel Osciak, 2016/02/26 01:51:25]

Honestly, I would really prefer not doing it this way. I understand this will
somewhat work, assuming only for OWNED_MEMORY *and* due to the fact how we
allocate (just one buffer) *and* how AlignedAlloc/Free work.

But this is very scary in case things change. And this is very, very non-obvious
to anyone changing this later on, for example, it's not obvious why this only
works for OWNED_MEMORY and how, if someone decides to extend this later; it's
not obvious that we may only be using one-buffer allocation and malloc/free that
stores size by itself, not using the sizes in VF, etc.

There is also the code complexity which I believe we should try not to introduce
to VF if we can help it. Could we for example use realloc outside of this class
and re-Wrap() the frame as different format?

+  bool DropYV12AAlphaChannel();
+
   // Adds a callback to be run when the VideoFrame is about to be destroyed.
   // The callback may be run from ANY THREAD, and so it is up to the client to
   // ensure thread safety.  Although read-only access to the members of this
@@ -464,7 +467,7 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
   void AllocateYUV(bool zero_initialize_memory);
 
   // Frame format.
-  const VideoPixelFormat format_;
+  VideoPixelFormat format_;
 
   // Storage type for the different planes.
   StorageType storage_type_;  // TODO(mcasas): make const