RESOURCE_UTILIZATION in VideoFrameMetadata, a consumer feedback signal.

RESOURCE_UTILIZATION in VideoFrameMetadata, a consumer feedback signal.

Adds a RESOURCE_UTILIZATION to media::VideoFrameMetadata which will allow
consumers to convey a "throttling feedback signal" to the producers of
media::VideoFrames.  Plumbs-in browser-side and renderer-side passing of
the metadata value across the IPC layer.

Also in this change, the media::cast::VideoSender sets the
RESOURCE_UTILIZATION metadata value to indicate CPU encode and network
bandwidth bottlenecks in the end-to-end system (sending a video stream
to a remote receiver).  An upcoming change will allow the screen capture
pipeline to use this feedback to automatically adjust resolution and
framerate during a session, maintaining the highest-quality user
experience possible in the current environment.  See bug for more
details.

BUG=156767

Review URL: https://codereview.chromium.org/1157193002

Cr-Commit-Position: refs/heads/master@{#333313}