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Adds a new FRAME_DURATION option to VideoFrameMetadata, which can be
used by consumers of video frames to improve performance (e.g., encoding
quality).

This change also adds population of the new metadata by the desktop/tab
capture pipeline, and consumption by Cast Streaming's software VP8
encoder.  Having accurate frame duration information improves the
encoder's ability to choose a compression quality level that better
meets the target encode bitrate.

Later changes will require this in order to compute resource utilization
feedback signals (see bug for details).

BUG=156767

Review URL: https://codereview.chromium.org/1146723002

Cr-Commit-Position: refs/heads/master@{#330661}