Mark virtual/softwarecompositing/.../webgl-with-accelerated-background-color.html failing after r162755.

This is a reftest that is producing an ImageOnlyFailure, but I'm not seeing
an obvious diff.

TBR=timloh@chromium.org, dongseong.hwang@intel.com, eseidel@chromium.org
BUG=324177

Review URL: https://codereview.chromium.org/93903002

git-svn-id: svn://svn.chromium.org/blink/trunk@162811 bbb929c8-8fbe-4397-9dbb-9b2b20218538