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@196152 bbb929c8-8fbe-4397-9dbb-9b2b20218538