media: Run mojo media application in GPU process on desktop by default

When "enable_mojo_media" is enabled (which is disabled by default on
desktop), we'll host the mojo media application in the gpu process
and have "video_decoder" service enabled. This will be used to test and
support the mojo video decoder service.

For the CDM service on desktop, since the CDM doesn't need to talk to
any other mojo services (it will be called directly by the media
pipeline in the render process), we'll use a different mojo URL (e.g.
mojo:cdm) and host it in the utility process separately.

BUG=522298,403462

Review-Url: https://codereview.chromium.org/2067073002
Cr-Commit-Position: refs/heads/master@{#399929}
3 files changed