Add MojoVideoDecoder.

This skeleton adds support for a MojoVideoDecoder, but does not
implement a decoder service.

To enable, use these gn args:
  enable_mojo_media = true
  mojo_media_services = [ "video_decoder" ]

BUG=522298

Review URL: https://codereview.chromium.org/1800953002

Cr-Commit-Position: refs/heads/master@{#381331}
9 files changed