Revert of DatabaseBackend objects should be destructed in the originator thread

Revert of DatabaseBackend objects should be destructed in the originator thread (https://codereview.chromium.org/183883005/)

Reason for revert:
Unnecessary change


Original issue's description:
> DatabaseBackend objects should be destructed in the originator thread.
> 
> Before this CL, it was possible that DatabaseThread held the last
> references of DatabaseBackend objects and it was released in the
> database thread.
> We must destruct objects in the originator thread in Oilpan.
> 
> Implementation:
> DatabaseThread::recordDatabaseClosed doesn't remove a RefPtr<
> DatabaseBackend>, and we release RefPtr<DatabaseBackend> objects
> in ~DatabaseThread, which is executed in the main/worker thread.
> Note that there are no ways to close a database explicitly, and
> DatabaseBackend objects are never destructed before their
> DatabaseContext is stopped. DatabaseThread::recordDatabaseClosed
> is actually unnecessary.
> 
> DatabaseThread doesn't know DatabaseBackend open status any longer.
> We remove DatabaseThread::isDatabaseOpen, and its existing
> callsites use DatabaseBackend::open() instead.
> 
> 
> BUG=347902
> TEST=none; no visible behavior changes hopefully.
> R=michaeln@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168362

TBR=michaeln@chromium.org,oilpan-reviews@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=347902
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168431

[tkent, 2014-03-05 04:59:03]

Created Revert of DatabaseBackend objects should be destructed in the originator
thread

[commit-bot: I haz the power, 2014-03-05 04:59:49]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tkent@chromium.org/178103007/1

[commit-bot: I haz the power, 2014-03-05 05:02:01]

Change committed as 168431