Switch blink::HTMLElement to use new downcast helpers (5/n)

Bug: 891908
Change-Id: I076f80229a09f8ff8faec8b6d677fd0e5f5310f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601621
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#658645}
12 files changed