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Previously, MCVD destroyed the textures that back the VideoFrames
when MCVD was destroyed.  When the pipeline suspends, this causes
any VideoFrame to become undrawable.

This CL adds TexturePool, which holds references to the textures
until all VideoFrames are destroyed.  While the implementation
isn't exactly a pool, we will be adding pool functionality to
it shortly.

This CL also introduces a wrapper for GL command buffer stubs, to
allow for easier testing.

Similarly, this CL adds a wrapper around TextureRef.

Bug: 737220
Test: TexturePoolTest
Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3d9bd42b9f8d3782b3eb4ceca30388774dc4854b
Reviewed-on: https://chromium-review.googlesource.com/966782
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545609}