media: scaffolding and plumbing for MojoRenderer{Impl, Service}
* Add skeleton client-side and service-side MediaRenderer pieces with basic plumbing and connections but no real logic.
* model a media::DemuxerStream in a mojom to clean up MediaRenderer interface and line-up with RendererImpl expectations.
* add DecoderConfig plumbing support
* add a basic unittest verifying things do actually hook up
* consolidate around 'media_mojo_' naming in media.gyp and add media_mojo target to build all this stuff.
BUG=410451
Review URL: https://codereview.chromium.org/
551963004
Cr-Commit-Position: refs/heads/master@{#295579}