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Cast was originally intended to tell vp8 to only reference buffers which have been ACKed.
However, that mode was buggy and disabled. This CL fixes it again. The three-buffer mode
lets the receiver drop frames and continue, which should be quite useful if we want lower
latency.

BUG=351596

Review URL: https://codereview.chromium.org/339743002

Cr-Commit-Position: refs/heads/master@{#293243}