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/667218Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>