Fix svg/masking/* tests.

"rx" is not a valid <circle> attribute - these tests were not drawing anything.

Also switching to rects to avoid reftest antialiasing issues.

For non-alpha variants, using a 50% luminance to capture the effect on the
output color.

R=pdr@chromium.org,dschulze@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23485004

git-svn-id: svn://svn.chromium.org/blink/trunk@156742 bbb929c8-8fbe-4397-9dbb-9b2b20218538