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In the GPU process, GpuArcVideoService (arc::VideoHost) is registered
via content's gpu client hook. It also manages additional IPC channel
per accelerator and dispatches calls to ArcVideoAccelerator.

In the browser process, GpuArcVideoServiceHost listens events of
ArcBridgeService and pass to GpuArcVideoService.

BUG=b/25057601
TEST=video from arc playbacks

Review URL: https://codereview.chromium.org/1641353003

Cr-Commit-Position: refs/heads/master@{#387799}