Introduce toSVGGradientElement(), use it
https://bugs.webkit.org/show_bug.cgi?id=118943

Reviewed by Andreas Kling.

As a step to change static_cast with toSVGXXX, static_cast<SVGGradientElement*> can
be changed with toSVGGradientElement().

No new tests, no behavior change.

* rendering/svg/RenderSVGGradientStop.cpp:
(WebCore::RenderSVGGradientStop::gradientElement):
* rendering/svg/RenderSVGResourceGradient.cpp:
(WebCore::RenderSVGResourceGradient::applyResource):
* rendering/svg/SVGResources.cpp:
(WebCore::targetReferenceFromResource):
* svg/SVGGradientElement.h:
(WebCore::toSVGGradientElement):
* svg/SVGLinearGradientElement.cpp:
(WebCore::SVGLinearGradientElement::collectGradientAttributes):
* svg/SVGRadialGradientElement.cpp:
(WebCore::SVGRadialGradientElement::collectGradientAttributes):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152955 268f45cc-cd09-0410-ab3c-d52691b4dbfc