Remove SVGElement::childShouldCreateRenderer

All implementation now return true, and the RenderObject::isChildAllowed
mechanism is used instead.

BUG=337607

Review URL: https://codereview.chromium.org/157663002

git-svn-id: svn://svn.chromium.org/blink/trunk@166733 bbb929c8-8fbe-4397-9dbb-9b2b20218538