Integrate registry_hash_store_contents with the rest of tracked prefs.

components/user_prefs/tracked/tracked_preferences_migration_unittest.cc

Index: components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
diff --git a/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc b/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
index 3c6258f0890fdcf1a16010231dfd0543f0490dc0..b963c36731b5c78d050d220664a29bdaedcd818e 100644
--- a/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
+++ b/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
@@ -47,9 +47,10 @@ class SimpleInterceptablePrefFilter : public InterceptablePrefFilter {
  public:
   // PrefFilter remaining implementation.
   void FilterUpdate(const std::string& path) override { ADD_FAILURE(); }
-  void FilterSerializeData(
+  base::Callback<void(bool)> FilterSerializeData(
       base::DictionaryValue* pref_store_contents) override {
     ADD_FAILURE();
+    return base::Callback<void(bool)>();
   }
 
  private:
@@ -162,10 +163,8 @@ class TrackedPreferencesMigrationTest : public testing::Test {
     DCHECK(store);
 
     base::StringValue string_value(value);
-    pref_hash_store
-        ->BeginTransaction(std::unique_ptr<HashStoreContents>(
-            new DictionaryHashStoreContents(store)))
-        ->StoreHash(key, &string_value);
+    DictionaryHashStoreContents contents(store);
+    pref_hash_store->BeginTransaction(&contents)->StoreHash(key, &string_value);
   }
 
   // Returns true if the store opposite to |store_id| is observed for its next