[IndexedDB] Removed FIXMEs as consistent enums being used on both sides of IPC.

Removed traces of typedefs from blink code:
blink::WebIDBCursor::Direction
blink::WebIDBDatabase::TaskType
blink::WebIDBDatabase::PutMode
blink::WebIDBDatabase::TransactionMode

As below patches rolled out to use consistent enums on browser and blink side.
Chromium side: https://codereview.chromium.org/320833002
Blink side: https://codereview.chromium.org/325683002

BUG=381848

Review URL: https://codereview.chromium.org/348203002

git-svn-id: svn://svn.chromium.org/blink/trunk@176689 bbb929c8-8fbe-4397-9dbb-9b2b20218538