Previously the ScrollView scrollbar and vertical borders did not span
across the entire horizontal dialog, with this change the scroll bar is
all the way to the right, and scrollview top/bottom dividers span past
dialog insets.

The insets are instead applied to the ScrollView contents, so the text
content is still inset properly. This Harmonizes dialogs that make use
of MessageBoxView, specifically JavaScript prompt/alert dialogs.

Bug: chromium:683346
Change-Id: Icbaa18b26a22975ba3fcf9d2b584416636cba07f
Reviewed-on: https://chromium-review.googlesource.com/767450


Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516145}