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

Bug: 891908
Change-Id: I5ddf6d1dcd8f281570236ca1e6432c97021e450a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638241
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667704}
20 files changed