When accessing line boxes, use RenderBlockFlow instead of RenderBlock.

Source/core/rendering/RenderBlock.h

Index: Source/core/rendering/RenderBlock.h
diff --git a/Source/core/rendering/RenderBlock.h b/Source/core/rendering/RenderBlock.h
index 6f41d8d610e8072860a5e9d772c3f6f665a71fa9..07109523001d0ef1b8fdcd13706ebdeaaac5a6f5 100644
--- a/Source/core/rendering/RenderBlock.h
+++ b/Source/core/rendering/RenderBlock.h
@@ -89,11 +89,16 @@ public:
 
     LayoutUnit minLineHeightForReplacedRenderer(bool isFirstLine, LayoutUnit replacedHeight) const;
 
+protected:
     RenderLineBoxList* lineBoxes() { return &m_lineBoxes; }
 
     InlineFlowBox* firstLineBox() const { return m_lineBoxes.firstLineBox(); }
     InlineFlowBox* lastLineBox() const { return m_lineBoxes.lastLineBox(); }
 
+    RootInlineBox* firstRootBox() const { return static_cast<RootInlineBox*>(firstLineBox()); }
+    RootInlineBox* lastRootBox() const { return static_cast<RootInlineBox*>(lastLineBox()); }
+
+public:
     // FIXME-BLOCKFLOW: Remove virtualizaion when all callers have moved to RenderBlockFlow
     virtual void deleteLineBoxTree();
 
@@ -160,9 +165,6 @@ public:
     LayoutUnit blockDirectionOffset(const LayoutSize& offsetFromBlock) const;
     LayoutUnit inlineDirectionOffset(const LayoutSize& offsetFromBlock) const;
 
-    RootInlineBox* firstRootBox() const { return static_cast<RootInlineBox*>(firstLineBox()); }
-    RootInlineBox* lastRootBox() const { return static_cast<RootInlineBox*>(lastLineBox()); }
-
     virtual bool shouldPaintSelectionGaps() const OVERRIDE FINAL;
     GapRects selectionGapRectsForRepaint(const RenderLayerModelObject* repaintContainer);
     LayoutRect logicalLeftSelectionGap(RenderBlock* rootBlock, const LayoutPoint& rootBlockPhysicalPosition, const LayoutSize& offsetFromRootBlock,