[Mac] Support undo in the omnibox.

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.
 
 #import "chrome/browser/cocoa/autocomplete_text_field.h"
 
 #include "base/logging.h"
 #import "chrome/browser/cocoa/autocomplete_text_field_cell.h"
 
 @implementation AutocompleteTextField
@@ skipping 142 matching lines @@
     // -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];
   }
 }
 
+- (NSUndoManager*)undoManagerForTextView:(NSTextView*)textView {
+  if (!undoManager_.get())
+    undoManager_.reset([[NSUndoManager alloc] init]);
+  return undoManager_.get();
+}
+
+- (void)clearUndoChain {
+  [undoManager_ removeAllActions];
+}
+
 @end