[LayoutNG] Refactor dialog positioning into NG

Bug: 740993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I5a0dd12b81fd300b42bfae0c7a5432ffadd6fb39
Reviewed-on: https://chromium-review.googlesource.com/1110488
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569532}
5 files changed