[New Multicolumn] Add support for column-span:all

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 95a00b5e2f49aa709e52da1aa5602b36db140356..9657aa21461dc6cd06c7273a28b6a9615e225ebb 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -359,6 +359,7 @@ public:
     virtual bool isRenderImage() const { return false; }
     virtual bool isRenderInline() const { return false; }
     virtual bool isRenderMultiColumnSet() const { return false; }
+    virtual bool isRenderMultiColumnSpannerPlaceholder() const { return false; }
     virtual bool isRenderPart() const { return false; }
     virtual bool isRenderRegion() const { return false; }
     virtual bool isRenderScrollbarPart() const { return false; }
@@ -490,7 +491,7 @@ public:
         // RenderBlock::createAnonymousBlock(). This includes creating an anonymous
         // RenderBlock having a BLOCK or BOX display. Other classes such as RenderTextFragment
         // are not RenderBlocks and will return false. See https://bugs.webkit.org/show_bug.cgi?id=56709.
-        return isAnonymous() && (style()->display() == BLOCK || style()->display() == BOX) && style()->styleType() == NOPSEUDO && isRenderBlock() && !isListMarker() && !isRenderFlowThread()
+        return isAnonymous() && (style()->display() == BLOCK || style()->display() == BOX) && style()->styleType() == NOPSEUDO && isRenderBlock() && !isListMarker() && !isRenderFlowThread() && !isRenderMultiColumnSet()

[rune, 2014/07/01 08:41:39]

I guess the added check here is because of
RenderMultiColumnSpannerPlaceholder::createAnonymous(). Then why isn't this
testing isRenderMultiColumnSpannerPlaceholder() ?

[mstensho (USE GERRIT), 2014/08/26 09:43:58]

It's here to make sure that a column set isn't treated as a "regular" anonymous
block (as defined in CSS 2.1). Regular anonymous blocks may be deleted, merged
and split when contents require it (take a look at what happens deep inside
RenderBlock's addChild() and removeChild()). We don't want any of that to happen
to column sets. A spanner placeholder isn't a RenderBlock, so it's not necessary
to test for it here.

[rune, 2014/08/27 07:00:01]

Acknowledged.

             && !isRenderFullScreen()
             && !isRenderFullScreenPlaceholder();
     }