Oilpan: Prepare to move IDBKey to Oilpan heap.

- IDBKeyRange::lower() and upper() are simple getters. We may return raw
  pointers.  We need to change WebIDBKeyRange.cpp too because of it.

- Remove |using RefCounted<IDBKey>::ref/deref| in IDBKey.h. They had
  no effect because of no other ref/deref.

- Remove unused IDBObjectStore::IndexKeyMap

BUG=366528

Review URL: https://codereview.chromium.org/255373002

git-svn-id: svn://svn.chromium.org/blink/trunk@172950 bbb929c8-8fbe-4397-9dbb-9b2b20218538