Fixes bug in paint scrollbar corner (again)

I think this got borked during a merge. Adding back.

TBR as this is same code as you reviewed.

BUG=341787
TEST=none
R=sadrul@chromium.org
TBR=sadrul@chromium.org

Review URL: https://codereview.chromium.org/165473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251203 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251203