Create a PrefStore in support of Blimp metrics collection.

components/prefs/BUILD.gn

 # Copyright 2015 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.
 
 component("prefs") {
   sources = [
     "default_pref_store.cc",
     "default_pref_store.h",
+    "in_memory_pref_store.cc",
+    "in_memory_pref_store.h",
     "json_pref_store.cc",
     "json_pref_store.h",
     "overlay_user_pref_store.cc",
     "overlay_user_pref_store.h",
     "pref_change_registrar.cc",
     "pref_change_registrar.h",
     "pref_member.cc",
     "pref_member.h",
     "pref_notifier_impl.cc",
     "pref_notifier_impl.h",
@@ skipping 59 matching lines @@
     "//base",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "default_pref_store_unittest.cc",
+    "in_memory_pref_store_unittest.cc",
     "json_pref_store_unittest.cc",
     "overlay_user_pref_store_unittest.cc",
     "pref_change_registrar_unittest.cc",
     "pref_member_unittest.cc",
     "pref_notifier_impl_unittest.cc",
     "pref_service_unittest.cc",
     "pref_value_map_unittest.cc",
     "pref_value_store_unittest.cc",
     "scoped_user_pref_update_unittest.cc",
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//base/test:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }