Blink side of adding a flag to enable the Scheduler.

Blink side of adding a flag to enable the Scheduler, the actual disabling logic
is expected to change after skyostil@'s refactors have landed.

Please note, this patch doesn't turn on anything new, rather it will allow
the scheduling logic to be turned off via a flag.


The chromium side is in https://codereview.chromium.org/625563002/

BUG=391005

Review URL: https://codereview.chromium.org/619323002

git-svn-id: svn://svn.chromium.org/blink/trunk@183260 bbb929c8-8fbe-4397-9dbb-9b2b20218538