Use lineageOfType to simplify two rendering helpers.
<https://webkit.org/b/126498>

Reviewed by Antti Koivisto.

* rendering/RenderRuby.cpp:
(WebCore::findRubyRunParent):
* rendering/svg/SVGRenderSupport.cpp:
(WebCore::SVGRenderSupport::findTreeRootObject):

    Simplify two functions that walk their parent chain to find the
    closest ancestor of a certain type.

* rendering/RenderRubyRun.h:

    Add requisite isRendererOfType<RenderRubyRun>().


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161323 268f45cc-cd09-0410-ab3c-d52691b4dbfc