IndexedDB: Remove IDBCursorBackendInterface/Proxy

WebCore can call directly into the blink::WebIDBCursor provided
by the embedder.

R=jamesr@chromium.org,alecflett@chromium.org,dgrogan@chromium.org
BUG=234277

Review URL: https://codereview.chromium.org/66833004

git-svn-id: svn://svn.chromium.org/blink/trunk@162048 bbb929c8-8fbe-4397-9dbb-9b2b20218538