[PhysicalWeb] Add physical_web namespace.

components/physical_web/data_source/physical_web_data_source_impl.h

 // Copyright 2016 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 COMPONENTS_PHYSICAL_WEB_DATA_SOURCE_PHYSICAL_WEB_DATA_SOURCE_IMPL_H_
 #define COMPONENTS_PHYSICAL_WEB_DATA_SOURCE_PHYSICAL_WEB_DATA_SOURCE_IMPL_H_
 
 #include "base/observer_list.h"
 #include "components/physical_web/data_source/physical_web_data_source.h"
 
+namespace physical_web {
+
 class PhysicalWebListener;
 
 class PhysicalWebDataSourceImpl : public PhysicalWebDataSource {
  public:
   PhysicalWebDataSourceImpl();
   ~PhysicalWebDataSourceImpl() override;
 
   // Register for changes to Physical Web URLs and associated page metadata.
   void RegisterListener(PhysicalWebListener* physical_web_listener) override;
 
   // Unregister for changes to Physical Web URLs and associated page metadata.
   void UnregisterListener(PhysicalWebListener* physical_web_listener) override;
 
   // Notify all registered listeners that a URL has been found.
   void NotifyOnFound(const std::string& url);
 
   // Notify all registered listeners that a URL has been lost.
   void NotifyOnLost(const std::string& url);
 
   // Notify all registered listeners that a distance has changed for a URL.
   void NotifyOnDistanceChanged(const std::string& url,
                                double distance_estimate);
 
  private:
   base::ObserverList<PhysicalWebListener> observer_list_;
 };
 
+}  // namespace physical_web
+
 #endif  // COMPONENTS_PHYSICAL_WEB_DATA_SOURCE_PHYSICAL_WEB_DATA_SOURCE_IMPL_H_