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

Bug: 891908
Change-Id: I8abcbef0a8be790a8c477c1e7b3c6fa1a9568f30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571265
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653140}
6 files changed