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

chrome/browser/cocoa/autocomplete_text_field.mm

Index: chrome/browser/cocoa/autocomplete_text_field.mm
diff --git a/chrome/browser/cocoa/autocomplete_text_field.mm b/chrome/browser/cocoa/autocomplete_text_field.mm
index 3071519b4e27235b6b74aa2257626ff292842330..182a8b32c0fe82dd40500ccef7159c622f973055 100644
--- a/chrome/browser/cocoa/autocomplete_text_field.mm
+++ b/chrome/browser/cocoa/autocomplete_text_field.mm
@@ -193,4 +193,19 @@
   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