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This patch adds EmitFrames() to VideoEncoder. The method is documented
as indicating that the encoder should produce all the frames that it may
be buffering for analysis.

This patch modifies VideoSender::InsertRawVideoFrame() to call
EmitFrames() when a frames is dropped because the in-flight frames
duration exceeds the target playout delay.

This patch provides an implementation of EmitFrames() for the
VideoToolbox H.264 encoder, which buffers frames for analysis as
described in the bug. It also uses the existing
max_number_of_video_buffers_used field in VideoConfig to set the
upper bound on the encoder's frame window.

BUG=450798
R=hclam, miu, DaleCurtis, Robert Sesek

Review URL: https://codereview.chromium.org/863083002

Cr-Commit-Position: refs/heads/master@{#313643}