IndexedDB: Constness sprinkles for some accessor methods

Sprinkle "const" around a few accessor methods. This also lets us
eliminate the 'keyPathAny' hack for IDBObjectStore, and CL 73313002
is taking care of that for IDBIndex.

R=alecflett@chromium.org

Review URL: https://codereview.chromium.org/59913009

git-svn-id: svn://svn.chromium.org/blink/trunk@162068 bbb929c8-8fbe-4397-9dbb-9b2b20218538