[mojo-edk] Add MojoWatch and MojoCancelWatch APIs

mojo/edk/system/message_pipe_dispatcher.h

Index: mojo/edk/system/message_pipe_dispatcher.h
diff --git a/mojo/edk/system/message_pipe_dispatcher.h b/mojo/edk/system/message_pipe_dispatcher.h
index f898ea8a46fad05cd12b4981beac21cda110f9e1..7400515f93036670ec130d3fbf61b20cd31d0fb6 100644
--- a/mojo/edk/system/message_pipe_dispatcher.h
+++ b/mojo/edk/system/message_pipe_dispatcher.h
@@ -42,6 +42,10 @@ class MessagePipeDispatcher : public Dispatcher {
   // Dispatcher:
   Type GetType() const override;
   MojoResult Close() override;
+  MojoResult Watch(MojoHandleSignals signals,
+                   const Watcher::WatchCallback& callback,
+                   uintptr_t context) override;
+  MojoResult CancelWatch(uintptr_t context) override;
   MojoResult WriteMessage(const void* bytes,
                           uint32_t num_bytes,
                           const DispatcherInTransit* dispatchers,