scheduler: Tell the scheduler about frame timing

This patch routes information about the current frame start time, the
deadline by which rendering should finish and the expected interval
between frames to the Blink scheduler. This information will be used to
schedule lower priority and idle tasks so that they do not preempt
input processing or painting.

BUG=391005

Review URL: https://codereview.chromium.org/516103003

git-svn-id: svn://svn.chromium.org/blink/trunk@181253 bbb929c8-8fbe-4397-9dbb-9b2b20218538