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

components/autofill/core/common/autofill_pref_names.cc

Index: components/autofill/core/common/autofill_pref_names.cc
diff --git a/components/autofill/core/common/autofill_pref_names.cc b/components/autofill/core/common/autofill_pref_names.cc
index 622783c0540c3e9478f3d90a8f125a4960a22f7e..d7385bb366b02091c4a2ff424e0459dfa904a606 100644
--- a/components/autofill/core/common/autofill_pref_names.cc
+++ b/components/autofill/core/common/autofill_pref_names.cc
@@ -12,6 +12,12 @@ namespace prefs {
 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";