Switch blink::CDATASection and blink::CharacterData to use new downcast helpers

Bug: 891908
Change-Id: Ib8ce35ef3ccfaf715ea25701acbd80cc5cbd875d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571267
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653145}
12 files changed