Need to roll back the multicol machinery state when re-laying out a block child.

third_party/WebKit/Source/core/layout/LayoutFlowThread.h

Index: third_party/WebKit/Source/core/layout/LayoutFlowThread.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutFlowThread.h b/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
index e0bf6709615f970bf692e256b4f9f1ab73a87b82..19a52b062223f24fe54f02fd61027ac892e36de7 100644
--- a/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
+++ b/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
@@ -41,6 +41,21 @@ class LayoutMultiColumnSet;
 
 typedef ListHashSet<LayoutMultiColumnSet*> LayoutMultiColumnSetList;
 
+// Layout state for multicol. To be stored when laying out a block child, so that we can roll back
+// to the initial state if we need to re-lay out said block child.
+class MultiColumnLayoutState {
+    friend class LayoutMultiColumnFlowThread;
+
+public:
+    MultiColumnLayoutState() : m_columnSet(nullptr) { }
+
+private:
+    explicit MultiColumnLayoutState(LayoutMultiColumnSet* columnSet) : m_columnSet(columnSet) { }
+    LayoutMultiColumnSet* columnSet() const { return m_columnSet; }
+
+    LayoutMultiColumnSet* m_columnSet;
+};
+
 // LayoutFlowThread is used to collect all the layout objects that participate in a flow thread. It
 // will also help in doing the layout. However, it will not layout directly to screen. Instead,
 // LayoutMultiColumnSet objects will redirect their paint and nodeAtPoint methods to this
@@ -88,6 +103,9 @@ public:
     virtual void contentWasLaidOut(LayoutUnit logicalBottomInFlowThreadAfterPagination) = 0;
     virtual bool canSkipLayout(const LayoutBox&) const = 0;
 
+    virtual MultiColumnLayoutState multiColumnLayoutState() const = 0;
+    virtual void restoreMultiColumnLayoutState(const MultiColumnLayoutState&) = 0;
+
     // Find and return the next logical top after |flowThreadOffset| that can fit unbreakable
     // content as tall as |contentLogicalHeight|. |flowThreadOffset| is expected to be at the exact
     // top of a column that's known to not have enough space for |contentLogicalHeight|. This method