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The new test page uses the viewport tag which makes it easy to sample
pixels from the screenshot. Adding the viewport tag to pixel_css3d page
which is also used for pixel tests because it starts to fail due to
unknown reasons on random bots.

BUG=368495

Review URL: https://codereview.chromium.org/302003015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274469 0039d316-1c4b-4281-b951-d872f2087c98