Disable some webgl conformance tests due to a security issue on video canvas painting

      Check http://crbug.com/334204 for details

BUG=334204,367962
R=alokp@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/251113005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266716 0039d316-1c4b-4281-b951-d872f2087c98