Plumbs through support for painting the scrollbarcorner

The corner is shown when there is both a vertical and horizontal
scrollbar. We need this as we want to draw the corner differently (a
different color) for win-aura now.

BUG=333499
TEST=none
R=ben@chromium.org

Review URL: https://codereview.chromium.org/146693015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247656 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247656