Switch base::Value typemapping to be by value instead of by unique_ptr.

services/preferences/public/cpp/pref_store_client_mixin.h

Index: services/preferences/public/cpp/pref_store_client_mixin.h
diff --git a/services/preferences/public/cpp/pref_store_client_mixin.h b/services/preferences/public/cpp/pref_store_client_mixin.h
index 19cf250c586ff1dfbe42c43cc300f21e52c1af00..1726fb3d562f47014f0259e5ec5c62615a89ea7c 100644
--- a/services/preferences/public/cpp/pref_store_client_mixin.h
+++ b/services/preferences/public/cpp/pref_store_client_mixin.h
@@ -45,7 +45,7 @@ class PrefStoreClientMixin : public BasePrefStore,
 
   // Initializes |this|. This must be called before any of the other public or
   // protected member functions.
-  void Init(std::unique_ptr<base::DictionaryValue> initial_prefs,
+  void Init(const base::DictionaryValue* initial_prefs,
             bool initialized,
             mojom::PrefStoreObserverRequest observer_request);
 
@@ -57,8 +57,7 @@ class PrefStoreClientMixin : public BasePrefStore,
   void OnPrefsChanged(std::vector<mojom::PrefUpdatePtr> updates) override;
   void OnInitializationCompleted(bool succeeded) override;
 
-  void OnPrefChanged(const std::string& key,
-                     std::unique_ptr<base::Value> value);
+  void OnPrefChanged(const std::string& key, base::Optional<base::Value> value);
 
   // Cached preferences.
   // If null, indicates that initialization failed.