cc: Use Scheduler::BeginFrame() in SingleThreadProxy
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}