Adds plumbing to pass WebUI mojo::Handle from browser to renderer

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'shared_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
@@ skipping 230 matching lines @@
       'include_dirs': [
         '..'
       ],
       'sources': [
         'public/bindings/allocation_scope.h',
         'public/bindings/array.h',
         'public/bindings/buffer.h',
         'public/bindings/callback.h',
         'public/bindings/error_handler.h',
         'public/bindings/interface.h',
+        'public/bindings/js/defaults.cc',
+        'public/bindings/js/defaults.h',
         'public/bindings/message.h',
         'public/bindings/passable.h',
         'public/bindings/remote_ptr.h',
         'public/bindings/sync_dispatcher.h',
         'public/bindings/type_converter.h',
         'public/bindings/lib/array.cc',
         'public/bindings/lib/array_internal.h',
         'public/bindings/lib/array_internal.cc',
         'public/bindings/lib/bindings_internal.h',
         'public/bindings/lib/bindings_serialization.cc',
@@ skipping 99 matching lines @@
         'mojo_bindings',
         'mojo_system',
       ],
       'export_dependent_settings': [
         'mojo_bindings',
         'mojo_system',
       ],
     },
   ],
 }