media: Pass output_cb to MojoMediaClient::CreateVideoDecoder()

This callback allows us to associate a ReleaseMailboxCB with each
output, without having to implement a remote interface for VideoFrames.
This exactly matches the semantics of VDA::ReusePictureBuffer, and will
be used to implement it, but also provides a place to stash TextureRef
objects for other decoders.

BUG=522298

Review-Url: https://codereview.chromium.org/2928043002
Cr-Commit-Position: refs/heads/master@{#479890}
6 files changed