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Adds support for long idle times in the Blink Scheduler.
A long idle time is initiated
if there are no frames being drawn and we are not in TOUCHSTART policy. Each
long idle period will be a maximum of 50 milliseconds or the time to the next
pending delayed task (whichever is sooner). Long idle periods will continually
be scheduled until a call to EndIdlePeriod is made. If there are no pending
tasks in the idle queue then the callback used to initate the next long idle
task will be posted on the control_task_after_wakeup_runner_ task runner, thus
ensuring that it won't cause the scheduler to wake up until another task
could have posted more idle work.

Currently, the long idle tasks are not enabled due to crbug.com/467655 -
once this is fixed they will be enabled.

Long idle task design doc:
https://docs.google.com/a/chromium.org/document/d/1yBlUdYW8VTIfB-DqhvQqUeP0kf-Ap1W4cao2yQq58Do/edit

BUG=455713,467655

Review URL: https://codereview.chromium.org/968073003

Cr-Commit-Position: refs/heads/master@{#320785}