Make childShouldCreateRenderer into an SVG only concept

The only callers are in SVG, and removing this hook will encourage new 
element developers to use Shadow DOM and CSS instead of this magical 
backdoor. By doing so we make it easier to produce components that can 
be transitioned to JS or made into web components.

In merging the logic from SVGDocument and SVGElement into
RenderTreeBuilder I was also able to fix the SVGTests feature so that
it correctly disables rendering again so now the following tests pass:
svg/dynamic-updates/SVGSVGElement-dom-requiredFeatures.html
svg/dynamic-updates/SVGSVGElement-svgdom-requiredFeatures.html

Note: svg/custom/system-language-crash.html test seems to be a whitespace
difference that doesn't matter.

Review URL: https://codereview.chromium.org/137623004

git-svn-id: svn://svn.chromium.org/blink/trunk@165670 bbb929c8-8fbe-4397-9dbb-9b2b20218538