[New Multicolumn] Create RenderMultiColumnSet during renderer creation, not during layout.

Source/core/rendering/RenderMultiColumnSet.h

Index: Source/core/rendering/RenderMultiColumnSet.h
diff --git a/Source/core/rendering/RenderMultiColumnSet.h b/Source/core/rendering/RenderMultiColumnSet.h
index d858b23b3d3e61af8d10de58bcd20fa0687dd414..98a68bfa14b2d9323c551de085dfee272016efee 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -32,24 +32,34 @@
 
 namespace WebCore {
 
-// RenderMultiColumnSet represents a set of columns that all have the same width and height. By combining runs of same-size columns into a single
-// object, we significantly reduce the number of unique RenderObjects required to represent columns.
+// RenderMultiColumnSet represents a set of columns that all have the same width and height. By
+// combining runs of same-size columns into a single object, we significantly reduce the number of
+// unique RenderObjects required to represent columns.
 //
-// A simple multi-column block will have exactly one RenderMultiColumnSet child. A simple paginated multi-column block will have three
-// RenderMultiColumnSet children: one for the content at the bottom of the first page (whose columns will have a shorter height), one
-// for the 2nd to n-1 pages, and then one last column set that will hold the shorter columns on the final page (that may have to be balanced
-// as well).
+// Column sets are inserted as anonymous children of the actual multicol container (i.e. the
+// renderer whose style computes to non-auto column-count and/or column-width).
 //
-// Column spans result in the creation of new column sets as well, since a spanning region has to be placed in between the column sets that
-// come before and after the span.
+// Being a "region", a column set has no children on its own, but is merely used to slice a portion
+// of the tall "single-column" flow thread into actual columns visually, to convert from flow thread
+// coordinates to visual ones. It is in charge of both positioning columns correctly relatively to
+// the parent multicol container, and to calculate the correct translation for each column's
+// contents, and to paint any rules between them. RenderMultiColumnSet objects are used for
+// painting, hit testing, and any other type of operation that requires mapping from flow thread
+// coordinates to visual coordinates.
+//
+// Column spans result in the creation of new column sets, since a spanning renderer has to be
+// placed in between the column sets that come before and after the span.
 class RenderMultiColumnSet FINAL : public RenderRegionSet {
 public:
-    static RenderMultiColumnSet* createAnonymous(RenderFlowThread*);
+    static RenderMultiColumnSet* createAnonymous(RenderFlowThread*, RenderStyle* parentStyle);
 
     virtual bool isRenderMultiColumnSet() const OVERRIDE { return true; }
 
     RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
     RenderMultiColumnFlowThread* multiColumnFlowThread() const { return multiColumnBlockFlow()->multiColumnFlowThread(); }
+
+    RenderMultiColumnSet* nextSiblingMultiColumnSet() const;
+
     unsigned computedColumnCount() const { return m_computedColumnCount; }
     LayoutUnit computedColumnWidth() const { return m_computedColumnWidth; }
     LayoutUnit computedColumnHeight() const { return m_computedColumnHeight; }