MediaService: Implement GetChannelToken IPC
This new IPC provides a non-forgeable token identifying a GpuChannel so
that it can be passed to Mojo services that want to share the same
channel in the GPU process (namely VDAv2 in the MojoMediaApplication).
For simplicity, the IPC is sync. It is handled on the IO thread in the
GPU process, so latency is expected to be manageable (and vastly better
that the already sync CreateVideoDecodeAccelerator IPC).
BUG=522298
Review-Url: https://codereview.chromium.org/2349463003
Cr-Commit-Position: refs/heads/master@{#419826}
2 files changed
