Switch blink::ShadowRoot to use new downcast helpers.

BUG=891908

Change-Id: I16d05f7adfb60be24d53942dc0a63a6189a2a806
Reviewed-on: https://chromium-review.googlesource.com/c/1481473
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634395}
22 files changed