MojoVideoDecoder: Plumb metadata methods.

This CL plumbs NeedsBitstreamConversion() and GetMaxDecodeRequests(),
which will be available after Initialize() completes, and
CanReadWithoutStalling(), which is updated each time a Decode()
completes.

BUG=522298

Review-Url: https://chromiumcodereview.appspot.com/2429723006
Cr-Commit-Position: refs/heads/master@{#426705}
5 files changed