Decouple lifetime of database thread and WebDatabase, chromium side.

      Add a method to WebDatabaseObserver to close a database without holding
on to an actual WebDatabase. This will make it possible to decouple the
lifetime of WebDatabase and the database thread.

BUG=249502
R=jam@chromium.org

Review URL: https://codereview.chromium.org/50883004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233367 0039d316-1c4b-4281-b951-d872f2087c98