Use tighter typing for RenderTextFragment::firstLetter()

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 4023 matching lines @@
     // FIXME: If textLength is 0, length may still be 1!
     return length;
 }
 
 void RenderBlock::createFirstLetterRenderer(RenderObject* firstLetterBlock, RenderObject* currentChild, unsigned length)
 {
     ASSERT(length && currentChild->isText());
 
     RenderObject* firstLetterContainer = currentChild->parent();
     RenderStyle* pseudoStyle = styleForFirstLetter(firstLetterBlock, firstLetterContainer);
-    RenderObject* firstLetter = 0;
+    RenderBoxModelObject* firstLetter = 0;
     if (pseudoStyle->display() == INLINE)
         firstLetter = RenderInline::createAnonymous(&document());
     else
         firstLetter = RenderBlockFlow::createAnonymous(&document());
     firstLetter->setStyle(pseudoStyle);
 
     // FIXME: The first letter code should not modify the render tree during
     // layout. crbug.com/370458
     DeprecatedDisableModifyRenderTreeStructureAsserts disabler;
 
@@ skipping 10 matching lines @@
     // This text fragment might be empty.
     RenderTextFragment* remainingText =
         new RenderTextFragment(textObj->node() ? textObj->node() : &textObj->document(), oldText.impl(), length, oldText.length() - length);
     remainingText->setStyle(textObj->style());
     if (remainingText->node())
         remainingText->node()->setRenderer(remainingText);
 
     firstLetterContainer->addChild(remainingText, textObj);
     firstLetterContainer->removeChild(textObj);
     remainingText->setFirstLetter(firstLetter);
-    toRenderBoxModelObject(firstLetter)->setFirstLetterRemainingText(remainingText);
+    firstLetter->setFirstLetterRemainingText(remainingText);
 
     // construct text fragment for the first letter
     RenderTextFragment* letter =
         new RenderTextFragment(remainingText->node() ? remainingText->node() : &remainingText->document(), oldText.impl(), 0, length);
     letter->setStyle(pseudoStyle);
     firstLetter->addChild(letter);
 
     textObj->destroy();
 }
 
@@ skipping 906 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