This completes the work to connect from a renderer to the VideoDecoder service in the GPU process, and implements a basic, functional VideoDecoder service.

There are no working VideoDecoders to actually instantiate yet, so the service will always return false from Configure(). Later CLs will add command buffer stub sharing and other requirements so that real decoders can be implemented.

BUG=522298

Review-Url: https://codereview.chromium.org/1899363002
Cr-Commit-Position: refs/heads/master@{#395971}