Initialize pref_model_associator_client_ with nullptr.

Some clients (in tests) doesn't call SetPrefModelAssociatorClient.
This leaves pref_model_associator_client_ in uninitialized state.
And CreateSyncable call creates PrefServiceSyncable with
invalid client_ pointer.

Change-Id: I89b310729ef817d75374d2fe66245d7538665e5b
Reviewed-on: https://chromium-review.googlesource.com/923968Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537645}

components/sync_preferences/pref_service_syncable_factory.h

@@ -50,7 +50,7 @@ class PrefServiceSyncableFactory : public PrefServiceFactory {
       std::unique_ptr<PrefValueStore::Delegate> delegate = nullptr);
 
  private:
-  PrefModelAssociatorClient* pref_model_associator_client_;
+  PrefModelAssociatorClient* pref_model_associator_client_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(PrefServiceSyncableFactory);
 };