Add alpha channel to VideoFrameBuffer containers

- Add alpha accessors to PlanarYuvBuffer interface, null by defualt.
- Add WrapI420ABuffer() that creates a container which implements these
accessors.
- Show the use via StereoDecoderAdapter.

This CL is the step 2 for adding alpha channel support over the wire in webrtc.
See https://webrtc-review.googlesource.com/c/src/+/7800 for the experimental
CL that gives an idea about how it will come together.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: Id5691cde00088ec811b63d89080d33ad2d6e3939
Reviewed-on: https://webrtc-review.googlesource.com/21130
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20635}

common_video/include/video_frame_buffer.h

@@ -78,6 +78,19 @@ rtc::scoped_refptr<I444BufferInterface> WrapI444Buffer(
     int v_stride,
     const rtc::Callback0<void>& no_longer_used);
 
+rtc::scoped_refptr<I420ABufferInterface> WrapI420ABuffer(
+    int width,
+    int height,
+    const uint8_t* y_plane,
+    int y_stride,
+    const uint8_t* u_plane,
+    int u_stride,
+    const uint8_t* v_plane,
+    int v_stride,
+    const uint8_t* a_plane,
+    int a_stride,
+    const rtc::Callback0<void>& no_longer_used);
+
 rtc::scoped_refptr<PlanarYuvBuffer> WrapYuvBuffer(
     VideoFrameBuffer::Type type,
     int width,