[layoutng] Handle overflow: auto more correctly

We need to relayout whenever scrollbars change. The change in
https://chromium-review.googlesource.com/1162657 was not quite correct.

The change to CopyFragmentDataToLayoutBox is necessary to not break
virtual/layout_ng_experimental/fragmentation/auto-overflow.html

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I589d6a1bd1b3f06224e6c4aeae3e7cdf163206cc
Reviewed-on: https://chromium-review.googlesource.com/1178804
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584316}
5 files changed