<https://webkit.org/b/120684>

Reviewed by Antti Koivisto.

Subtree relayout scheduling should only happen while there's a render tree up,
the code can freely assume that there's a RenderView present.

Added an assertion that the render tree isn't being torn down. This should catch
renderers doing unnecessary work during document detach and could be a source
of "performance freebies."

I also un-nested scheduleRelayoutOfSubtree() with early-return style to make
the code more human-readable.

* page/FrameView.h:
* page/FrameView.cpp:
(WebCore::FrameView::scheduleRelayoutOfSubtree):
* rendering/RenderObject.cpp:
(WebCore::RenderObject::scheduleRelayout):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155046 268f45cc-cd09-0410-ab3c-d52691b4dbfc