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

Review URL: https://codereview.chromium.org/399383005

git-svn-id: svn://svn.chromium.org/blink/trunk@178644 bbb929c8-8fbe-4397-9dbb-9b2b20218538