Introduce a SVGUnknownElement class for unknown SVG elements

Introduce a SVGUnknownElement class and have it override rendererIsNeeded()
to return false. We now use SVGUnknownElement type (instead of SVGElement)
for unknown SVG elements. As a consequence, SVGElement::rendererIsNeeded()
no longer needs to return false and can finally behave as
SVGStyledElement::rendererIsNeeded() used to behave.

R=pdr
BUG=231536

Review URL: https://chromiumcodereview.appspot.com/21498002

git-svn-id: svn://svn.chromium.org/blink/trunk@155426 bbb929c8-8fbe-4397-9dbb-9b2b20218538