Web Animations: Timeline should not advance during task execution

Previously the animation clock was only frozen during animation frame
callbacks. This meant that the timeline was able to advance during other
tasks. 

This patch allows the clock to advance once per task, either to the
compositor supplied frame start time or to an approximation of the next
expected frame time.

http://dev.w3.org/fxtf/web-animations/#script-execution-and-live-updates-to-the-model

BUG=367903

Review URL: https://codereview.chromium.org/251183006

git-svn-id: svn://svn.chromium.org/blink/trunk@173583 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Speculative fix for crash accessing a supercluster root.

BUG=369485

Review URL: https://codereview.chromium.org/274463002

git-svn-id: svn://svn.chromium.org/blink/trunk@173584 bbb929c8-8fbe-4397-9dbb-9b2b20218538