DatabaseManager::terminateDatabaseThread should handle multiple DatabaseContexts.

      This is a followup of Blink r190021 [1].

[1] http://src.chromium.org/viewvc/blink?view=rev&rev=190021

BUG=455368,455789

Review URL: https://codereview.chromium.org/913413002

git-svn-id: svn://svn.chromium.org/blink/trunk@190035 bbb929c8-8fbe-4397-9dbb-9b2b20218538