IndexedDB: Release cursor backend when cursor is finished

Free up cursor resources ASAP, rather than waiting for GC to claim
the cursor object first. Also, move the IDBRequest->IDBCursor
link breaking to the most specific onSuccess() variant. Also,
inline the trivial IDBCursor accessors (not worth tracing).

R=alecflett@chromium.org,dgrogan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19693008

git-svn-id: svn://svn.chromium.org/blink/trunk@154762 bbb929c8-8fbe-4397-9dbb-9b2b20218538