[layoutng] Don't copy data to legacy for intermediate layouts

In some places, notably the min/max content calculation, we produce layout
results that are just for intermediate use, they are not final fragments
for a box. We should avoid creating paint fragments for them or copying
data to the legacy tree.

This becomes an issue in practice with fragment caching, because we may never
reach the code to produce the final fragment and fix up the paint fragment,
etc.

R=kojii@chromium.org,eae@chromium.org

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I012b599685a0f67060e6225c195e0e8205afad8e
Reviewed-on: https://chromium-review.googlesource.com/1064855
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559718}
7 files changed