[LayoutNG] Create NGContainerFragmentBuilder.

This captures the common methods that are needed between
NGFragmentBuilder, and NGLineBoxFragmentBuilder.

I didn't place them into NGBaseFragmentBuilder, as text doesn't really
need them.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I5fe83a94021890bbe0a6a4e096a1aa2893138899
Reviewed-on: https://chromium-review.googlesource.com/706343
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508981}
16 files changed