Generate toSVGGradient|Graphics|PolyElement()

This cl generate toSVGFoo() for SVGGradientElement, SVGGraphicsElement, and SVGPolyElement().
We can use more plenty toSVGFoo() functions, and help to detect bad type casts.

BUG=309516

Review URL: https://codereview.chromium.org/105753003

git-svn-id: svn://svn.chromium.org/blink/trunk@163215 bbb929c8-8fbe-4397-9dbb-9b2b20218538