Pref service: Filter updates from read-only pref stores.

services/preferences/public/cpp/pref_store_client_mixin.cc

 // Copyright 2017 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 "services/preferences/public/cpp/pref_store_client_mixin.h"
 
 #include <utility>
 
 #include "base/values.h"
 #include "services/preferences/public/cpp/pref_store_client.h"
@@ skipping 84 matching lines @@
     if (cached_prefs_->Get(key, &prev)) {
       if (!prev->Equals(value.get())) {
         cached_prefs_->Set(key, std::move(value));
         changed = true;
       }
     } else {
       cached_prefs_->Set(key, std::move(value));
       changed = true;
     }
   }
-  if (changed)
+  if (changed && initialized_)
     ReportPrefValueChanged(key);
 }
 
 template <typename BasePrefStore>
 void PrefStoreClientMixin<BasePrefStore>::OnInitializationCompleted(
     bool succeeded) {
   if (!initialized_) {
     initialized_ = true;
     for (auto& observer : observers_)
       observer.OnInitializationCompleted(succeeded);
   }
 }
 
 template class PrefStoreClientMixin<::PrefStore>;
 template class PrefStoreClientMixin<::PersistentPrefStore>;
 
 }  // namespace prefs