Makes scroll corner anti-aliased for test runner

Apparently old code was doing this, so needed for image comparison
layout test.

BUG=333499
TEST=none
R=abarth@chromium.org
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/154013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248760 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248760