Use tighter typing for RenderTextFragment::firstLetter()

Source/core/editing/TextIterator.cpp

Index: Source/core/editing/TextIterator.cpp
diff --git a/Source/core/editing/TextIterator.cpp b/Source/core/editing/TextIterator.cpp
index c62fb7e13f968d93416ab829a008d2ebdcd67def..60dc3cb6d24ddcf2b0778fa03d1f8b8ab3e65d53 100644
--- a/Source/core/editing/TextIterator.cpp
+++ b/Source/core/editing/TextIterator.cpp
@@ -743,7 +743,7 @@ void TextIterator::handleTextBox()
     }
 }
 
-static inline RenderText* firstRenderTextInFirstLetter(RenderObject* firstLetter)
+static inline RenderText* firstRenderTextInFirstLetter(RenderBoxModelObject* firstLetter)
 {
     if (!firstLetter)
         return 0;
@@ -759,7 +759,7 @@ static inline RenderText* firstRenderTextInFirstLetter(RenderObject* firstLetter
 void TextIterator::handleTextNodeFirstLetter(RenderTextFragment* renderer)
 {
     if (renderer->firstLetter()) {
-        RenderObject* r = renderer->firstLetter();
+        RenderBoxModelObject* r = renderer->firstLetter();
         if (r->style()->visibility() != VISIBLE && !m_ignoresStyleVisibility)
             return;
         if (RenderText* firstLetter = firstRenderTextInFirstLetter(r)) {