Add explicit initialization of TestingPrefServiceSimple pref stores

This helper method is needed to fully complete TestingPrefServiceSimple
initialization in unit tests.

Bug: 992889
Change-Id: I752b2a3333375177448cbe37e4945e08cfdbbdc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810917
Commit-Queue: Nikita Podguzov <nikitapodguzov@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697644}

components/prefs/testing_pref_service.h

@@ -64,6 +64,9 @@ class TestingPrefServiceBase : public SuperPrefService {
   // Do-nothing implementation for TestingPrefService.
   static void HandleReadError(PersistentPrefStore::PrefReadError error) {}
 
+  // Set initialization status of pref stores.
+  void SetInitializationCompleted();
+
  protected:
   TestingPrefServiceBase(TestingPrefStore* managed_prefs,
                          TestingPrefStore* extension_prefs,
@@ -230,4 +233,14 @@ void TestingPrefServiceBase<SuperPrefService, ConstructionPrefRegistry>::
   pref_store->RemoveValue(path, WriteablePrefStore::DEFAULT_PREF_WRITE_FLAGS);
 }
 
+template <class SuperPrefService, class ConstructionPrefRegistry>
+void TestingPrefServiceBase<SuperPrefService, ConstructionPrefRegistry>::
+    SetInitializationCompleted() {
+  managed_prefs_->SetInitializationCompleted();
+  extension_prefs_->SetInitializationCompleted();
+  recommended_prefs_->SetInitializationCompleted();
+  // |user_prefs_| is initialized in PrefService constructor so no need to
+  // set initialization status again.
+}
+
 #endif  // COMPONENTS_PREFS_TESTING_PREF_SERVICE_H_