Refactoring: Add toWebFrameImpl() interface.

Source/web/mac/WebSubstringUtil.mm

 /*
  * Copyright (C) 2005, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  * 
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 39 matching lines @@
 #include "core/rendering/RenderObject.h"
 #include "core/rendering/style/RenderStyle.h"
 #include "public/platform/WebRect.h"
 
 using namespace WebCore;
 
 namespace WebKit {
 
 NSAttributedString* WebSubstringUtil::attributedSubstringInRange(WebFrame* webFrame, size_t location, size_t length)
 {
-    Frame* frame = static_cast<WebFrameImpl*>(webFrame)->frame();
+    Frame* frame = toWebFrameImpl(webFrame)->frame();
     if (frame->view()->needsLayout())
         frame->view()->layout();
 
     RefPtr<Range> range(TextIterator::rangeFromLocationAndLength(frame->selection()->rootEditableElementOrDocumentElement(), location, length));
     if (!range)
         return nil;
 
     NSMutableAttributedString* string = [[NSMutableAttributedString alloc] init];
     NSMutableDictionary* attrs = [NSMutableDictionary dictionary];
 
@@ skipping 35 matching lines @@
                                            length:characters.size()] autorelease];
         [string replaceCharactersInRange:NSMakeRange(position, 0)
                               withString:substring];
         [string setAttributes:attrs range:NSMakeRange(position, numCharacters)];
         position += numCharacters;
     }
     return [string autorelease];
 }
 
 } // namespace WebKit