Add 2 GpuMemoryBuffers-specific owners in //media

This CL adds dcastagna@ and reveman@ as OWNERS for //media
*gpu_memory_buffer* files, namely:

$ find media | grep gpu_memory_buffer
media/capture/video/chromeos/local_gpu_memory_buffer_manager.h
media/capture/video/chromeos/mock_gpu_memory_buffer_manager.h
media/capture/video/chromeos/local_gpu_memory_buffer_manager.cc
media/capture/video/chromeos/mock_gpu_memory_buffer_manager.cc
media/video/gpu_memory_buffer_video_frame_pool.cc
media/video/gpu_memory_buffer_video_frame_pool.h
media/video/mock_gpu_memory_buffer_video_frame_pool.cc
media/video/mock_gpu_memory_buffer_video_frame_pool.h
media/video/gpu_memory_buffer_video_frame_pool_unittest.cc

(Since those files are spread, I added the rule in the top-level file).

Change-Id: I469f5177db8c170a6b79d483e4a812610be1b74b
Reviewed-on: https://chromium-review.googlesource.com/892159Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532936}

media/OWNERS

@@ -21,4 +21,8 @@ xhwang@chromium.org
 # For Fuchsia-specific changes:
 per-file *_fuchsia*=file://build/fuchsia/OWNERS
 
+# For GpuMemoryBuffer-related changes:
+per-file *gpu_memory_buffer*=dcastagna@chromium.org
+per-file *gpu_memory_buffer*=reveman@chromium.org
+
 # COMPONENT: Internals>Media