Convert the regions percentage margins pixel tests to reference tests.

BUG=264198
R=jchaffraix@chromium.org, ojan@chromium.org, eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/20179005

git-svn-id: svn://svn.chromium.org/blink/trunk@154971 bbb929c8-8fbe-4397-9dbb-9b2b20218538