Introduce a layering in the invalidation component as public and impl

sync/tools/null_invalidation_state_tracker.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_TOOLS_NULL_INVALIDATION_STATE_TRACKER_H_
 #define SYNC_TOOLS_NULL_INVALIDATION_STATE_TRACKER_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
-#include "components/invalidation/invalidation_state_tracker.h"
+#include "components/invalidation/impl/invalidation_state_tracker.h"
 
 namespace syncer {
 
 class NullInvalidationStateTracker
     : public base::SupportsWeakPtr<NullInvalidationStateTracker>,
       public InvalidationStateTracker {
  public:
   NullInvalidationStateTracker();
   ~NullInvalidationStateTracker() override;
 
   void ClearAndSetNewClientId(const std::string& data) override;
   std::string GetInvalidatorClientId() const override;
 
   std::string GetBootstrapData() const override;
   void SetBootstrapData(const std::string& data) override;
 
   void SetSavedInvalidations(const UnackedInvalidationsMap& states) override;
   UnackedInvalidationsMap GetSavedInvalidations() const override;
 
   void Clear() override;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_TOOLS_NULL_INVALIDATION_STATE_TRACKER_H_