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

Bug: 891908
Change-Id: I1e197c2d9e88d2bd9d2aa4a13a0d6fa37d5514d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565736
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#650307}
12 files changed