Revert of Convert remaining RenderSVG* nodes to RenderObject::isChildAllowed

Revert r165851 and r166238 because of performance regressions (issue 339470).

BUG=339470
TBR=pdr,schenney

Review URL: https://codereview.chromium.org/147213004

git-svn-id: svn://svn.chromium.org/blink/trunk@166392 bbb929c8-8fbe-4397-9dbb-9b2b20218538