Switch core/layout/ng/ to use downcast helpers

The following classes were switched over:

- NGPhysicalBoxFragment
- NGBlockBreakToken
- NGBoxFragment
- NGBlockNode
- NGPhysicalContainerFragment

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

BUG=891908

Change-Id: I5edfead9cfe91c254210de3c37f1bff832809d26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535480
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643390}
39 files changed