[LayoutNG] Change NGFragment to accept a reference instead of a pointer.

This also scopes down any NGFragment related classes to the minimum which
is needed.

Changes a few other related pieces of code to use const references.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I1ed13ce254aef4723c393877b5190295dcd12146
Reviewed-on: https://chromium-review.googlesource.com/646752
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#499295}
15 files changed