[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 615432111823054f598e7f2684635d703c08b0fd..cf561af36e7e84626a6db715d591796ebeb1a2a1 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -392,6 +392,7 @@ 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; }
@@ -533,7 +534,7 @@ 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()
             && !isRenderFullScreen()
             && !isRenderFullScreenPlaceholder();
     }