Make the embedder API use the ChannelManager.

mojo/edk/embedder/test_embedder.h

Index: mojo/edk/embedder/test_embedder.h
diff --git a/mojo/edk/embedder/test_embedder.h b/mojo/edk/embedder/test_embedder.h
index b6203b41b0cdba318c069ccdc8cabd42d9d01be2..76917ee539edaec74bc3094df1da2da4920ec26a 100644
--- a/mojo/edk/embedder/test_embedder.h
+++ b/mojo/edk/embedder/test_embedder.h
@@ -20,6 +20,9 @@ MOJO_SYSTEM_IMPL_EXPORT void InitWithSimplePlatformSupport();
 // do more work to ensure that tests don't leak, etc.) Returns true if there
 // were no problems, false if there were leaks -- i.e., handles still open -- or
 // any other problems.
+//
+// Note: It is up to the caller to ensure that there are not outstanding
+// callbacks from |CreateChannel()| before calling this.
 MOJO_SYSTEM_IMPL_EXPORT bool Shutdown();
 
 }  // namespace test