[PhysicalWeb] Add physical_web namespace.

components/physical_web/data_source/physical_web_listener.h

Index: components/physical_web/data_source/physical_web_listener.h
diff --git a/components/physical_web/data_source/physical_web_listener.h b/components/physical_web/data_source/physical_web_listener.h
index eb338872f9a6aeac4247f45ea2b83ac471fbe4a5..910737e80a592f6eb50b8f247ff002715d37e2c3 100644
--- a/components/physical_web/data_source/physical_web_listener.h
+++ b/components/physical_web/data_source/physical_web_listener.h
@@ -7,6 +7,8 @@
 
 #include <string>
 
+namespace physical_web {
+
 // Class for being notified when Physical Web data changes.
 class PhysicalWebListener {
  public:
@@ -23,4 +25,6 @@ class PhysicalWebListener {
                                  double distance_estimate) = 0;
 };
 
+}  // namespace physical_web
+
 #endif // COMPONENTS_PHYSICAL_WEB_DATA_SOURCE_PHYSICAL_WEB_LISTENER_H_