IndexedDB: Use public API enums rather than matching enums/shorts

The public API uses "unsigned short" for cursor direction and
transaction mode, despite having an enum for one of them already. And
Blink also has a matching internal enum, for no particularly good
reason. Add the missing public enum and temp methods (until chromium
is updated) and drop the internal enums.

Review URL: https://codereview.chromium.org/137973005

git-svn-id: svn://svn.chromium.org/blink/trunk@166331 bbb929c8-8fbe-4397-9dbb-9b2b20218538