Enforce SVG content model for the switch element.

Most SVG elements have a restriction that they cannot contain "text
content child elements" as immediate children. See this spec section:
https://svgwg.org/svg2-draft/single-page.html#intro-TermTextContentElement

This patch enforces this rule for the SVG switch element as well. The
default SVGElement::childShouldCreateRenderer requires that no "text
content child elements" are children and this patch adds a call to
check SVGElement::childShouldCreateRenderer for the switch element.

This patch also contains a trivial change in SVGAElement to call
SVGGraphicsElement::childShouldCreateRenderer (its superclass) instead
of SVGElement::childShouldCreateRenderer. No change in behavior, but
this may prevent future bugs.

Additionally, I checked the other SVG childShouldCreateRenderer
functions for similar bugs and did not find any.

BUG=327626

Review URL: https://codereview.chromium.org/117913002

git-svn-id: svn://svn.chromium.org/blink/trunk@164089 bbb929c8-8fbe-4397-9dbb-9b2b20218538