Remove external begin frame source parameter and settings

This removes the external begin frame source setting and establishes
that the use output surface begin frame source is the only path
to providing begin frame sources.

This also removes all of the plumbing in the scheduler and the proxies
to pass along the external begin frame source in favor of just having
the CompositorExternalBeginFrameSource set it on the scheduler.

TBR=nyquist@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2324273002
Cr-Commit-Position: refs/heads/master@{#419935}