[CSSRegions] RenderFlowThread should not be created as a Document renderer
https://bugs.webkit.org/show_bug.cgi?id=113658

Reviewed by Julien Chaffraix.

After https://bugs.webkit.org/show_bug.cgi?id=107138, RenderFlowThread should not be constructed as a renderer for Document.
Since it does not have an element associated, it will be constructed using the pattern for anonymous blocks.
However, since I do not want the flow thread to be collapsed as anonymous blocks are, i changed RenderObject::isAnonymousBlock
to return false for a RenderFlowThread (the alternative would have been to patch more call sites to prevent RenderFlowThread
collapsing as anonymous block).

Covered by existing tests.

* rendering/FlowThreadController.cpp:
(WebCore::FlowThreadController::ensureRenderFlowThreadWithName):
* rendering/RenderFlowThread.cpp:
(WebCore::RenderFlowThread::RenderFlowThread):
* rendering/RenderFlowThread.h:
* rendering/RenderMultiColumnBlock.cpp:
(WebCore::RenderMultiColumnBlock::addChild):
* rendering/RenderMultiColumnFlowThread.cpp:
(WebCore::RenderMultiColumnFlowThread::RenderMultiColumnFlowThread):
(WebCore::RenderMultiColumnFlowThread::createAnonymous):
* rendering/RenderMultiColumnFlowThread.h:
(RenderMultiColumnFlowThread):
* rendering/RenderNamedFlowThread.cpp:
(WebCore::RenderNamedFlowThread::createAnonymous):
(WebCore::RenderNamedFlowThread::RenderNamedFlowThread):
* rendering/RenderNamedFlowThread.h:
(RenderNamedFlowThread):
* rendering/RenderObject.h:
(WebCore::RenderObject::isAnonymousBlock):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147414 268f45cc-cd09-0410-ab3c-d52691b4dbfc