HTMLFrameOwnerElement::getSVGDocument should return null rather than throwing.

If the element's 'contentDocument' is not an SVG document (or does not exist)
we currently throw a NotSupportedError exception. This matches neither the
spec[1] nor Firefox's implementation.

This CL drops the exception. The new test passes in Firefox (with the
exception of HTMLFrameElement::getSVGDocument, which Firefox doesn't
implement).

[1]: http://www.w3.org/TR/SVG/struct.html#InterfaceGetSVGDocument

BUG=270033

Review URL: https://codereview.chromium.org/85423003

git-svn-id: svn://svn.chromium.org/blink/trunk@162632 bbb929c8-8fbe-4397-9dbb-9b2b20218538