[layoutng] Remove an unnecessary duplicate call to ComputePadding

Also, makes CalculateBorderScrollbarPadding equivalent to
ComputeBorder + ComputePadding + GetScrollbarSizes.

This also makes the padding set on the fragment more correct.

R=eae@chromium.org

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I8db38ca5955d30fa789a636e5c6deb2ec1fd1829
Reviewed-on: https://chromium-review.googlesource.com/1169321
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582272}
3 files changed