Fix bad casts in BindingPointer::set. The object owned by the BindingPointer does not necessarily derive from RefCountObject; it could also just derive from RefCountObjectNoID (e.g. Compiler). Found with Clang's CFI bad cast checker. BUG=chromium:507755 Change-Id: I7e431746b2783e2fc0f2d347a4a27bd60da18473 Reviewed-on: https://chromium-review.googlesource.com/667218 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>