so that we get consistent results.  Also bumped up the pdiff threshold for 2d sample
on the mac to its previous number to get the test to pass until we fix it.
Review URL: http://codereview.chromium.org/155885

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@21223 0039d316-1c4b-4281-b951-d872f2087c98