Revert of Implement timers by posting delayed tasks (patchset #21 id:400001 of https://codereview.chromium.org/1134523002/)

      Reason for revert:
Reverting once again because this makes ScriptContextSetTest.LifeCycle leak:

http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_asan_rel_ng/builds/12840/steps/extensions_unittests%20%28with%20patch%29/logs/stdio

The reason is that many of these tests do things backwards: they first initialize Blink and then create a message loop. This means we can't create a real scheduler for Blink and have to jump through hoops to make things work. We could extend this mock scheduler to properly clean things up, but I think I'll instead try to get rid of the mock scheduler completely.

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196308

TBR=alexclarke@chromium.org,erikcorry@chromium.org,jochen@chromium.org,kinuko@chromium.org,rmcilroy@chromium.org,sigbjornf@opera.com,jchaffraix@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143,416362,480522

Review URL: https://codereview.chromium.org/1162903005

git-svn-id: svn://svn.chromium.org/blink/trunk@196317 bbb929c8-8fbe-4397-9dbb-9b2b20218538