Introduce internal::ServiceRegistry to prepare for ServiceProvider split.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
@@ skipping 376 matching lines @@
     },
     {
       'target_name': 'mojo_application',
       'type': 'static_library',
       'sources': [
         'public/cpp/application/application.h',
         'public/cpp/application/connect.h',
         'public/cpp/application/lib/application.cc',
         'public/cpp/application/lib/service_connector.cc',
         'public/cpp/application/lib/service_connector.h',
+        'public/cpp/application/lib/service_registry.cc',
+        'public/cpp/application/lib/service_registry.h',
       ],
       'dependencies': [
         'mojo_service_provider_bindings',
       ],
       'export_dependent_settings': [
         'mojo_service_provider_bindings',
       ],
     },
   ],
   'conditions': [
@@ skipping 15 matching lines @@
           },
           'dependencies': [
             'mojo_public_java',
           ],
           'includes': [ '../build/java.gypi' ],
         },
       ],
     }],
   ],
 }