Element should not override childShouldCreateRenderer

Instead of overriding this method we can just put the check in
RenderTreeBuilder itself. This paves the way to making
childShouldCreateRenderer an SVG only concept. Once my other patches land
we can guard the call to childShouldCreateRenderer with isSVGElement()
too.

Review URL: https://codereview.chromium.org/125653002

git-svn-id: svn://svn.chromium.org/blink/trunk@164569 bbb929c8-8fbe-4397-9dbb-9b2b20218538