[SVG2] Merge SVGStyledElement into SVGElement

Merge SVGStyledElement into SVGElement as per the latest SVG2 specification:
https://svgwg.org/svg2-draft/single-page.html#types-InterfaceSVGElement

This greatly simplifies the inheritance model in SVG and is a big step
towards supporting SVG2.

BUG=231536

Review URL: https://chromiumcodereview.appspot.com/21042009

git-svn-id: svn://svn.chromium.org/blink/trunk@155264 bbb929c8-8fbe-4397-9dbb-9b2b20218538