Refactor PrefHashStore's BeginTransaction to take a rawptr

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..3340649fe7d55b85739ff3cf3794d75f58ca5831 100644
--- a/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
+++ b/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
@@ -162,10 +162,8 @@ 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