Switch blink::Element to use new downcast helpers (1/n)

Bug: 891908
Change-Id: Ic360daeaf226f8f0fb7f36313cfd4c6500749915
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627351
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#664289}
19 files changed