Introduce toSVGLinearGradientElement(), and use it

Introduce toSVGLinearGradientElement(), and use it
https://bugs.webkit.org/show_bug.cgi?id=120154

Reviewed by Andreas Kling.

As a step to clean-up static_cast<SVGXXX>, toSVGLinearGradientElement() is added to clean-up
static_cast<SVGLinearGradientElement*>.

* rendering/svg/RenderSVGResourceLinearGradient.cpp:
(WebCore::RenderSVGResourceLinearGradient::collectGradientAttributes):
* rendering/svg/SVGRenderTreeAsText.cpp:
(WebCore::writeSVGResourceContainer):
* svg/SVGLinearGradientElement.cpp:
(WebCore::SVGLinearGradientElement::collectGradientAttributes):
* svg/SVGLinearGradientElement.h:
(WebCore::toSVGLinearGradientElement):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154472 268f45cc-cd09-0410-ab3c-d52691b4dbfc