Web Animations CSS: Delay animation-start event until notification from compositor.

This aligns the timing of start events under Web Animations with the legacy
implementation. Prior to this patch Web Animations would dispatch the start
event for a compositor animation before the start time was known.

This patch moves the skipping of redundant timing updates from Player to
TimedItem allowing updateInheritedTime to be aware of when a Player gains
a start time.

This opens up opportunities to further simplify updateInheritedTime, but they
have not been attempted in this patch to ease merging this fix.

BUG=333059

Review URL: https://codereview.chromium.org/133153003

git-svn-id: svn://svn.chromium.org/blink/trunk@165030 bbb929c8-8fbe-4397-9dbb-9b2b20218538