Switch blink::RemoteDOMWindow to use downcast helpers

R=jbroman@chromium.org
CC=blink-reviews-onion-soup@chromium.org

BUG=891908

Change-Id: I6d13142d63c924632370a1abd57b4effdb596f3c
Reviewed-on: https://chromium-review.googlesource.com/c/1483570
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#634812}
4 files changed