Introduce toSVGMaskElement(), and use it
https://bugs.webkit.org/show_bug.cgi?id=119443

Reviewed by Andreas Kling.

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

Blink merge from https://src.chromium.org/viewvc/blink?view=rev&revision=155432

* rendering/svg/RenderSVGResourceMasker.cpp:
(WebCore::RenderSVGResourceMasker::applyResource):
(WebCore::RenderSVGResourceMasker::resourceBoundingBox):
* rendering/svg/RenderSVGResourceMasker.h:
* svg/SVGMaskElement.h:
(WebCore::toSVGMaskElement):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153687 268f45cc-cd09-0410-ab3c-d52691b4dbfc