Add embedder::AsyncWait()

mojo/edk/embedder/embedder.h

Index: mojo/edk/embedder/embedder.h
diff --git a/mojo/edk/embedder/embedder.h b/mojo/edk/embedder/embedder.h
index 02b4ac3f0c904f9fcc9dad00b24d1a495bb85e64..987c68799a248a2f746fb5b1e50822516f3e32ea 100644
--- a/mojo/edk/embedder/embedder.h
+++ b/mojo/edk/embedder/embedder.h
@@ -117,6 +117,15 @@ MOJO_SYSTEM_IMPL_EXPORT MojoResult
 PassWrappedPlatformHandle(MojoHandle platform_handle_wrapper_handle,
                           ScopedPlatformHandle* platform_handle);
 
+// Start waiting the handle asynchronously. On success, |callback| will be
+// called exactly once, when |handle| satisfies a signal in |signals| or it
+// becomes known that it will never do so. |callback| will be executed on an
+// arbitrary thread. It must not call any Mojo system or embedder functions.
+MOJO_SYSTEM_IMPL_EXPORT MojoResult
+AsyncWait(MojoHandle handle,
+          MojoHandleSignals signals,
+          base::Callback<void(MojoResult)> callback);
+
 }  // namespace embedder
 }  // namespace mojo