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

chrome/browser/cocoa/autocomplete_text_field.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/cocoa/autocomplete_text_field.h"
 
 #include "base/logging.h"
 #include "chrome/browser/cocoa/autocomplete_text_field_cell.h"
 
 @implementation AutocompleteTextField
@@ skipping 175 matching lines @@
   if (observer_) {
     observer_->OnFrameChanged();
   }
 }
 
 // Due to theming, parts of the field are transparent.
 - (BOOL)isOpaque {
   return NO;
 }
 
+- (void)setAttributedStringValue:(NSAttributedString*)aString {
+  NSTextView* editor = static_cast<NSTextView*>([self currentEditor]);
+  if (!editor) {
+    [super setAttributedStringValue:aString];
+  } else {
+    // -currentEditor is defined to return NSText*, make sure our
+    // assumptions still hold, here.
+    DCHECK([editor isKindOfClass:[NSTextView class]]);
+
+    NSTextStorage* textStorage = [editor textStorage];
+    DCHECK(textStorage);
+    [textStorage setAttributedString:aString];
+  }
+}
+
 @end