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Previously MojoAndroidOverlays could only be created on the GPU main
thread. Now we can create them on any thread, and specifically the media
service thread, so that MediaCodecVideoDecoder can use them.

Also, they now embed an optional ServiceContextRef so that if they're
used from the media service, they can extend the lifetime of the thread
their mojo interface is bound to. This is necessary because they will
be jointly owned by both MCVD and CodecImages. 

Bug: 660942
Cq-Include-Trybots: 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: I6497b81eeb7205a4893ef6c4de4ee5af571ac90a
Reviewed-on: https://chromium-review.googlesource.com/684005Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Chris Watkins <watk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504854}