[Mac] Adjust Omnibox implementation to not break undo chain.

chrome/browser/autocomplete/autocomplete_edit_view_mac.mm

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/autocomplete/autocomplete_edit_view_mac.h"
 
 #include <Carbon/Carbon.h>  // kVK_Return
 
 #include "app/clipboard/clipboard.h"
 #include "app/gfx/font.h"
@@ skipping 412 matching lines @@
       color = InsecureSchemeColor();
       // Add a strikethrough through the scheme.
       [as addAttribute:NSStrikethroughStyleAttributeName
                  value:[NSNumber numberWithInt:NSUnderlineStyleSingle]
                  range:ComponentToNSRange(parts.scheme)];
     }
     [as addAttribute:NSForegroundColorAttributeName value:color
                range:ComponentToNSRange(parts.scheme)];
   }
 
-  [field_ setObjectValue:as];
+  [field_ setAttributedStringValue:as];
 
   // TODO(shess): This may be an appropriate place to call:
   //   controller_->OnChanged();
   // In the current implementation, this tells LocationBarViewMac to
   // mess around with |model_| and update |field_|.  Unfortunately,
   // when I look at our peer implementations, it's not entirely clear
   // to me if this is safe.  SetText() is sort of an utility method,
   // and different callers sometimes have different needs.  Research
   // this issue so that it can be added safely.
 
@@ skipping 450 matching lines @@
   // TODO(shess): Figure out where the selection belongs.  On GTK,
   // it's set to the start of the text.
 }
 
 // Signal that we've lost focus when the window resigns key.
 - (void)windowDidResignKey:(NSNotification*)notification {
   edit_view_->OnDidResignKey();
 }
 
 @end