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Webkit has css properties through which custom scroll bars can be added.
In case the element's visibility property is hidden and custom scrollbar 
properties are applied the scrollbar width is not considered when querying
the element.clientWidth. In case of non-custom scrollbars whether the
element's visibility property is hidden or not correct scrollbar width is
considered.

When a custom scrollbar is created there is a check whether the renderer to
which scrollbar is added is visible or not. This check is not required since
for non-custom scrollbars the same check is not present. Also whether the
element's visibility property is hidden or not a placeholder is set for the
element.

Both behavior i.e custom and non-custom scrollbars should be same.

I already landed this patch on Webkit. Just modified the layout test file.
http://trac.webkit.org/changeset/155323

R=esprehn@chromium.org,eseidel@chromium.org
BUG=343521

Review URL: https://codereview.chromium.org/163383002

git-svn-id: svn://svn.chromium.org/blink/trunk@175123 bbb929c8-8fbe-4397-9dbb-9b2b20218538