Switch t_p/blink/renderer/core/css away from DEFINE_CSS_RULE_TYPE_CASTS

... to use downcast helpers

BUG=891908

R=jbroman@chromium.org
CC=blink-reviews-vendor@chromium.org

Change-Id: I73216ed5fadbd095498e98613e089cea1b7514fb
Reviewed-on: https://chromium-review.googlesource.com/c/1494680
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636835}
22 files changed