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Proxy should use BeginFrame() to request new BeginFrame instead of BeginImplFrame().
The scheduler turns BeginFrames into BeginImplFrames or BeginRetroFrames
as necessary to make sure we don't start a new BeginImplFrame
before the previous deadline.

Some of public methods that are only used by itself moved into the private.

R=brianderson@chromium.org
BUG=NONE
TEST=cc_unittests

Review URL: https://codereview.chromium.org/536123002

Cr-Commit-Position: refs/heads/master@{#293259}