[Autofill] Add metrics to determine whether the system AddressBook is accessible.

components/autofill/core/common/autofill_pref_names.cc

 // Copyright 2013 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 "components/autofill/core/common/autofill_pref_names.h"
 
 namespace autofill {
 namespace prefs {
 
 // Boolean that is true when auxiliary Autofill profiles are enabled.
 // Currently applies to Address Book "me" card on Mac.  False on Win and Linux.
 const char kAutofillAuxiliaryProfilesEnabled[] =
     "autofill.auxiliary_profiles_enabled";
 
+// Boolean that is true when at least one read request for auxiliary Autofill
+// profiles has been issued.  Currently applies only to auxiliary profiles on
+// Mac.
+const char kAutofillAuxiliaryProfilesQueried[] =
+    "autofill.auxiliary_profiles_queried";
+
 // Boolean that is true if Autofill is enabled and allowed to save profile data.
 const char kAutofillEnabled[] = "autofill.enabled";
 
 // Double that indicates negative (for not matched forms) upload rate.
 const char kAutofillNegativeUploadRate[] = "autofill.negative_upload_rate";
 
 // Double that indicates positive (for matched forms) upload rate.
 const char kAutofillPositiveUploadRate[] = "autofill.positive_upload_rate";
 
 }  // namespace prefs
 }  // namespace autofill