[LayoutNG] remove unused TriggerScrollbars from ConstraintSpace

Unfortunatelly, handling scrollbars in NG is delayed to next milestone.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I50ba6eb28ed040da2cbb8857c1b3eb84c3a609a3
Reviewed-on: https://chromium-review.googlesource.com/1053298
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557432}
4 files changed