Switch blink::Element to use new downcast helpers (4/n)

Bug: 891908
Change-Id: I22d98b649a20730554af9fa33ae185e8ef76cb21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638478
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667719}
20 files changed