[New Multicolumn] Let a spanner's containing block be the multicol container.

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index d08b3e586e7a1029c8b6f54e29794111935075af..d3f05df2fad17f08318a9231172df9c9e4b3fdb1 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -52,6 +52,7 @@
 #include "core/rendering/RenderLayer.h"
 #include "core/rendering/RenderListBox.h"
 #include "core/rendering/RenderListMarker.h"
+#include "core/rendering/RenderMultiColumnSpannerPlaceholder.h"
 #include "core/rendering/RenderTableCell.h"
 #include "core/rendering/RenderView.h"
 #include "core/rendering/compositing/RenderLayerCompositor.h"
@@ -99,6 +100,15 @@ RenderBox::RenderBox(ContainerNode* node)
     setIsBox();
 }
 
+void RenderBox::willBeRemovedFromTree()
+{
+    if (m_rareData && m_rareData->m_spannerPlaceholder) {
+        m_rareData->m_spannerPlaceholder->spannerWillBeRemoved();
+        m_rareData->m_spannerPlaceholder = 0;
+    }
+    RenderBoxModelObject::willBeRemovedFromTree();
+}
+
 void RenderBox::willBeDestroyed()
 {
     clearOverrideSize();
@@ -1658,6 +1668,14 @@ void RenderBox::deleteLineBoxWrapper()
     }
 }
 
+void RenderBox::setSpannerPlaceholder(RenderMultiColumnSpannerPlaceholder* placeholder)

[Julien - ping for review, 2014/12/11 19:09:37]

We could maybe split setSpannerPlaceHolder (with ASSERT(placeholder) vs
clearSpannerPlaceholder.

This function is not too complicated but it does have 2 clear separated callers.

[mstensho (USE GERRIT), 2014/12/11 21:01:14]

Done.

+{
+    if (!placeholder && !m_rareData)
+        return;
+    RELEASE_ASSERT(!placeholder || !m_rareData || !m_rareData->m_spannerPlaceholder); // not expected to change directly from one spanner to another.
+    ensureRareData().m_spannerPlaceholder = placeholder;
+}
+
 LayoutRect RenderBox::clippedOverflowRectForPaintInvalidation(const RenderLayerModelObject* paintInvalidationContainer, const PaintInvalidationState* paintInvalidationState) const
 {
     if (style()->visibility() != VISIBLE) {