Coverity: Fix a few pass-by-values.

chrome/browser/autofill/autofill_external_delegate.cc

Index: chrome/browser/autofill/autofill_external_delegate.cc
diff --git a/chrome/browser/autofill/autofill_external_delegate.cc b/chrome/browser/autofill/autofill_external_delegate.cc
index 491ffee76157b18ddf2f831d6fc4ea5671cd04a1..891b10cc2cfdee8092fe6a2d0faa14081cc22d42 100644
--- a/chrome/browser/autofill/autofill_external_delegate.cc
+++ b/chrome/browser/autofill/autofill_external_delegate.cc
@@ -138,9 +138,8 @@ void AutofillExternalDelegate::DidEndTextFieldEditing() {
   has_shown_autofill_popup_for_current_edit_ = false;
 }
 
-void AutofillExternalDelegate::DidAcceptAutofillSuggestions(string16 value,
-                                                            int unique_id,
-                                                            unsigned index) {
+void AutofillExternalDelegate::DidAcceptAutofillSuggestions(
+    const string16& value, int unique_id, unsigned index) {
   // If the selected element is a warning we don't want to do anything.
   if (unique_id < 0)
     return;