Implement timers by posting delayed tasks
This patch refactors TimerBase to post tasks delayed tasks and
deletes the now-obsolete timer heap and shared timer mechanism.
ATTN Sheriffs: If there are weird layout test flakes all of a
sudden, this patch may be the cause since the interleaving of
timers with other posted tasks will change.
Original patch by Alex Clarke <alexclarke@chromium.org>.
BUG=463143,416362,480522
Review URL: https://codereview.chromium.org/
1134523002
git-svn-id: svn://svn.chromium.org/blink/trunk@196308 bbb929c8-8fbe-4397-9dbb-9b2b20218538