Allow media track switching.

media/video/gpu_memory_buffer_video_frame_pool.h

Index: media/video/gpu_memory_buffer_video_frame_pool.h
diff --git a/media/video/gpu_memory_buffer_video_frame_pool.h b/media/video/gpu_memory_buffer_video_frame_pool.h
index ed4b3617e53cd39b0cf78d7f7dd7e6b9d16ddd47..cf4ce3bf0e04b29b254c1e7504a4af6fedf22abc 100644
--- a/media/video/gpu_memory_buffer_video_frame_pool.h
+++ b/media/video/gpu_memory_buffer_video_frame_pool.h
@@ -24,6 +24,9 @@ class GpuVideoAcceleratorFactories;
 // The pool recycles resources to a void unnecessarily allocating and
 // destroying textures, images and GpuMemoryBuffer that could result
 // in a round trip to the browser/GPU process.
+// NOTE: Destroying the pool will not immediately invalidate outstanding video
+// frames. GPU memory buffers are ref-counted and will be kept alive for as long

[Daniele Castagna, 2017/03/31 00:58:13]

nit: Actually VideoFrames are ref-counted. The resources backing VideoFrames
stay alive until all VideoFrames are returned from the compositor. This includes
GMBs, images and textures.

[servolk, 2017/03/31 01:04:19]

Thanks for correcting me. How about this?
// NOTE: Destroying the pool will not immediately invalidate outstanding video
// frames. GPU memory buffers will be kept alive by video frames referencing
// them. Video frames themselves are ref-counted and will be released when they
// are no longer needed, potentially after the pool is destroyed.

[Daniele Castagna, 2017/03/31 01:20:54]

Thanks to you for adding this note.

The comment seems good. Maybe just remove the end of the second sentence where
you mention that video frames are referencing GMBs (just remove the "referencing
them" part), since it might seem they reference them directly.

The real story is that the release callbacks of videoframes are keeping alive
another ref counted object that contains all the resources needed. But I don't
think that belongs to this note.

I'm also OK adding an "indirectly" referencing them.

Thanks!

[servolk, 2017/03/31 01:23:21]

Done.

+// as the corresponding video frame is alive.
 class MEDIA_EXPORT GpuMemoryBufferVideoFramePool {
  public:
   GpuMemoryBufferVideoFramePool();