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This CL adds a CreateVideoEncodeAccelerator() methods to the gpu.mojom and
gpu_service.mojom files (renderer-to-browser interface and browser-to-gpu
interfaces, resp.), and wires the new mojom::VideoEncodeAcceleratorRequest
pass through.

This is step 0 of VEA mojification, which is currently IPC-based. More
info can be found in the DD https://goo.gl/PbTKnt, concretely this
CL can be better understood in the context of the first diagram in the
document, also here, for conveniency: https://goo.gl/HkwYvA.

A playground with most of the changes pre-review can be found in 
https://crrev.com/c/558846.

Bug: 736517
Change-Id: Id1f6d60d487d97ce3f304bead73738b2ba531408
Reviewed-on: https://chromium-review.googlesource.com/558833
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485550}