[New Multicolumn] Faster and more direct RenderMultiColumnSet::multiColumnFlowThread().

Source/core/rendering/RenderMultiColumnSet.h

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 #ifndef RenderMultiColumnSet_h
 #define RenderMultiColumnSet_h
 
+#include "core/rendering/RenderMultiColumnFlowThread.h"
 #include "core/rendering/RenderRegionSet.h"
 #include "wtf/Vector.h"
 
 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.
 //
 // 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).
 //
 // 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*, RenderStyle* parentStyle);
 
     virtual bool isRenderMultiColumnSet() const OVERRIDE { return true; }
 
     RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
-    RenderMultiColumnFlowThread* multiColumnFlowThread() const { return multiColumnBlockFlow()->multiColumnFlowThread(); }
+    RenderMultiColumnFlowThread* multiColumnFlowThread() const
+    {
+        ASSERT_WITH_SECURITY_IMPLICATION(!flowThread() || flowThread()->isRenderMultiColumnFlowThread());
+        return static_cast<RenderMultiColumnFlowThread*>(flowThread());
+    }
 
     RenderMultiColumnSet* nextSiblingMultiColumnSet() const;
 
     unsigned computedColumnCount() const { return m_computedColumnCount; }
     LayoutUnit computedColumnWidth() const { return m_computedColumnWidth; }
     LayoutUnit computedColumnHeight() const { return m_computedColumnHeight; }
 
     void setComputedColumnWidthAndCount(LayoutUnit width, unsigned count)
     {
         m_computedColumnWidth = width;
@@ skipping 109 matching lines @@
     };
     Vector<ContentRun, 1> m_contentRuns;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSet, isRenderMultiColumnSet());
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnSet_h