Close the WebSQL tracker db earlier on during shutdown and reliably on appropriate thread.

BUG=310381

Review URL: https://codereview.chromium.org/38433004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230623 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230623