Remove isRenderSVGBlock

It is only used once and can be replaced by using isSVG().

Review URL: https://codereview.chromium.org/137303004

git-svn-id: svn://svn.chromium.org/blink/trunk@165018 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Generate toSVGFooElement() for SVGFELight|AnimateMotion|FontFaceSrc|TextPositioningElement

As a step to use toSVGFooElement(), this cl generate toSVGFooElement() for mentioned SVG Element.
This will help to detect bad type cast.

BUG=309516

Review URL: https://codereview.chromium.org/133203003

git-svn-id: svn://svn.chromium.org/blink/trunk@165019 bbb929c8-8fbe-4397-9dbb-9b2b20218538