IndexedDB: Remove shim methods now that Chromium is updated to use enums

Following Chromium r249829 these temporary methods are no longer needed.

Review URL: https://codereview.chromium.org/153443003

git-svn-id: svn://svn.chromium.org/blink/trunk@166889 bbb929c8-8fbe-4397-9dbb-9b2b20218538