media: Implement MojoVideoDecoder::CanReadWithoutStalling().

This implementation is quite conservative; it only allows changes at the
same time as outputting a VideoFrame. This is fine, since the value is
irrelevant before the first frame, and the only dynamic case
(GpuVideoDecoder) changes values exactly on output.

BUG=522298

Review-Url: https://codereview.chromium.org/2951233002
Cr-Commit-Position: refs/heads/master@{#481989}
4 files changed