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 7cc9d6d625b4493b60946db3a29bbe02c3cdaec0..86f764a45b39a6255cb583efd07d2ec0c6cc3840 100644
--- a/Source/core/rendering/RenderBlock.h
+++ b/Source/core/rendering/RenderBlock.h
@@ -81,11 +81,16 @@ 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();
 
@@ -152,9 +157,6 @@ 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,