[LayoutNG] Add NGPhysicalContainerFragment

This patch adds NGPhysicalContainerFragment, a fragment that has
children, to share code between NGPhysicalBoxFragment and
NGPhysicalLineBoxFragment.

Bug: 591099
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ic72db2cf818f24e79402138caa8e885607dca591
Reviewed-on: https://chromium-review.googlesource.com/655018
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500359}
8 files changed