When accessing line boxes, use RenderBlockFlow instead of RenderBlock.

Source/core/rendering/RenderBlock.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 4201 matching lines @@
 static bool shouldCheckLines(RenderObject* obj)
 {
     return !obj->isFloatingOrOutOfFlowPositioned()
         && obj->isRenderBlock() && obj->style()->height().isAuto()
         && (!obj->isDeprecatedFlexibleBox() || obj->style()->boxOrient() == VERTICAL);
 }
 
 static int getHeightForLineCount(RenderBlock* block, int l, bool includeBottom, int& count)
 {
     if (block->style()->visibility() == VISIBLE) {
-        if (block->childrenInline()) {
-            for (RootInlineBox* box = block->firstRootBox(); box; box = box->nextRootBox()) {
+        if (block->isRenderBlockFlow() && block->childrenInline()) {
+            for (RootInlineBox* box = toRenderBlockFlow(block)->firstRootBox(); box; box = box->nextRootBox()) {
                 if (++count == l)
                     return box->lineBottom() + (includeBottom ? (block->borderBottom() + block->paddingBottom()) : LayoutUnit());
             }
-        }
-        else {
+        } else {
             RenderBox* normalFlowChildWithoutLines = 0;
             for (RenderBox* obj = block->firstChildBox(); obj; obj = obj->nextSiblingBox()) {
                 if (shouldCheckLines(obj)) {
                     int result = getHeightForLineCount(toRenderBlock(obj), l, false, count);
                     if (result != -1)
                         return result + obj->y() + (includeBottom ? (block->borderBottom() + block->paddingBottom()) : LayoutUnit());
-                } else if (!obj->isFloatingOrOutOfFlowPositioned())
+                } else if (!obj->isFloatingOrOutOfFlowPositioned()) {
                     normalFlowChildWithoutLines = obj;
+                }
             }
             if (normalFlowChildWithoutLines && l == 0)
                 return normalFlowChildWithoutLines->y() + normalFlowChildWithoutLines->height();
         }
     }
 
     return -1;
 }
 
 RootInlineBox* RenderBlock::lineAtIndex(int i) const
@@ skipping 692 matching lines @@
 void RenderBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const RenderObject* obj) const
 {
     showRenderObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace WebCore