cc: Add SetNeedsOneBeginImplFrame to the Scheduler

This requests another BeginImplFrame to happen. Once
the requests for new ones stop coming, the scheduler
will put the begin frame source to sleep.

This will be used to replace SetNeedsAnimate to
request new frames while an animation wants to tick.

BUG=522658
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1428133003

Cr-Commit-Position: refs/heads/master@{#357667}

• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_state_machine.cc[diff]
• cc/scheduler/scheduler_state_machine.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]