Use the scheduling mechanism provided by the platform

This uses the newly added WebScheduler interface for posting idle
tasks and checking whether main thread work should yield to let
more critical tasks run. This interface is implemented by the platform.

Design doc: https://docs.google.com/a/chromium.org/document/d/16f_RIhZa47uEK_OdtTgzWdRU0RFMTQWMpEWyWXIpXUo/edit#heading=h.srz53flt1rrp

BUG=391005

Review URL: https://codereview.chromium.org/656463004

git-svn-id: svn://svn.chromium.org/blink/trunk@184374 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Cleanup after http://crrev.com/623813002/ now the chrome side has landed.

BUG=383125

Review URL: https://codereview.chromium.org/680503004

git-svn-id: svn://svn.chromium.org/blink/trunk@184375 bbb929c8-8fbe-4397-9dbb-9b2b20218538