Fix a crashing bug that can happen during browser shutdown. If shutdown happens while task to commit changes to the db occurs the program crashes. The fix is to detect when we're in the STORING state and to return early.

BUG=330917,331918

Review URL: https://codereview.chromium.org/178733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253449 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253449