Revert of DatabaseBackend should not post a task to an already terminated database thread (patchset #4 id:60001 of https://codereview.chromium.org/596083005/)

Reason for revert:
This causes a lot of database layout test failures on the buildbots. Reverting.

Original issue's description:
> DatabaseBackend should not post a task to an already terminated database thread
> 
> Currently some methods of DatabaseBackend post a task to a database thread without checking DatabaseThread::terminationRequested(). Consequently, the task can be posted to an already terminated database thread.
> 
> This is problematic but by accident isn't currently causing any real issue (for some reason). This begins to cause an issue when I destruct DatabaseThread::m_thread promptly after the database thread processed a cleanup task (See https://codereview.chromium.org/589363002).
> 
> This CL adds the necessary DatabaseThread::terminationRequested()s before DatabaseBackend posts a task to the database thread.
> 
> BUG=340522
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182552

TBR=oilpan-reviews@chromium.org,tkent@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=340522

Review URL: https://codereview.chromium.org/593423002

git-svn-id: svn://svn.chromium.org/blink/trunk@182564 bbb929c8-8fbe-4397-9dbb-9b2b20218538