Scheduler: Don't shut down the scheduler too soon

Scheduler: Don't shut down the scheduler too soon

We shouldn't terminate the scheduler before ensuring background
threads such as the HTML parser have been stopped. This is
because those threads may schedule new work, leading to a crash
as we're trying to post tasks into a non-existing scheduler.

This patch moves the call to tear down the scheduler right
before terminating blink's Platform abstraction and right after
terminating WTF, as WTF also has a dependency on the scheduler
(callOnMain).

BUG=395420
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178644

[Sami, 2014-07-21 14:14:19]

PTAL Eric. Let me see if I can figure out a way to test this -- probably pretty
hard to do reliably.

[Sami, 2014-07-22 10:32:08]

Jochen, can you have a look? I don't think there's a reasonable way to write a
targeted test for this regression, but given that ASan/clusterfuzz tests are
failing left and right because of this, I think we're covered :)

[jochen (gone - plz use gerrit), 2014-07-22 10:54:13]

lgtm

[Sami, 2014-07-22 11:12:25]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2014-07-22 11:12:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/skyostil@chromium.org/399383005/1

[commit-bot: I haz the power, 2014-07-22 12:07:23]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/18235)

[commit-bot: I haz the power, 2014-07-22 12:45:39]

Change committed as 178644