Unify Preferences Mojom Naming to PreferencesService

services/preferences/public/cpp/BUILD.gn

 # Copyright 2016 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.
 
 source_set("cpp") {
   sources = [
-    "pref_observer_store.cc",
-    "pref_observer_store.h",
+    "pref_client_store.cc",
+    "pref_client_store.h",
   ]
 
   public_deps = [
     "//base",
     "//components/prefs:prefs",
     "//services/preferences/public/interfaces",
     "//services/service_manager/public/cpp",
   ]
 
   deps = [
     "//mojo/public/cpp/bindings:bindings",
   ]
 }