Move supplemental transform to rare data

The AffineTransformation for animateMotion is pretty rare
and just makes SVGGraphicsElement take up more memory, so
move it to SVGElementRareData.

Review URL: https://codereview.chromium.org/660653004

git-svn-id: svn://svn.chromium.org/blink/trunk@183854 bbb929c8-8fbe-4397-9dbb-9b2b20218538