IndexedDB: Resolve some 'remove this when...' TODOs

Remove some temporary code/uncomment some DCHECKs that were sprinkled
in the backend code while chasing down leaks several months ago.

R=alecflett@chromium.org

Review URL: https://codereview.chromium.org/25935002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227858 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227858