Vertical scrollbar width becomes zero in case of overlay scrollbar.

Chrome on Mac OS is having overlay scrollbars. So, we need to take care of the verticalScrollbarWidth() if we have overlay scrollbars.

BUG=335285

Review URL: https://codereview.chromium.org/138543008

git-svn-id: svn://svn.chromium.org/blink/trunk@166316 bbb929c8-8fbe-4397-9dbb-9b2b20218538