Oilpan: DatabaseThread::m_thread should not get destructed during sweeping

WebThreadSupportingGC's destructor has a safe point scope
because the destructor can block until all the tasks in the WebThread are processed.
This means that the WebThreadSupportingGC shouldn't get destruction during sweeping
where we cannot enter a safe point scope.

Currently DatabaseThread::m_thread is destructed during sweeping and causes the above issue.
To avoid the issue, this CL destructs the DatabaseThread::m_thread in DatabaseThread::terminate()
(which is called when DatabaseContext stops the database thread). That way we can prevent
the DatabaseThread::m_thread from getting destructed during sweeping.

BUG=416772

Review URL: https://codereview.chromium.org/589363002

git-svn-id: svn://svn.chromium.org/blink/trunk@182650 bbb929c8-8fbe-4397-9dbb-9b2b20218538