[LayoutNG] Move PrepareLayout to NGBlockNode::FirstChild

This isn't ideal, but it does make things faster.

A few more tests pass (don't timeout) only 1-2 additional failure.
So a net win?

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Id95bdd4ebcfd2534dbbf07b50a730c9961349713
Reviewed-on: https://chromium-review.googlesource.com/726846
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510224}
6 files changed