Revert of Enable multiprocess by default on desktop mojo_runner.

mojo/message_pump/handle_watcher.cc

Index: mojo/message_pump/handle_watcher.cc
diff --git a/mojo/message_pump/handle_watcher.cc b/mojo/message_pump/handle_watcher.cc
index 7dec3f39bd3f65237c52085be1c09cf607f767f0..964f1e2d3348d80b1edef376538ce2eac0862639 100644
--- a/mojo/message_pump/handle_watcher.cc
+++ b/mojo/message_pump/handle_watcher.cc
@@ -222,13 +222,7 @@
 }
 
 WatcherThreadManager* WatcherThreadManager::GetInstance() {
-  // We need to leak this because otherwise when the process dies, AtExitManager
-  // waits for destruction which waits till the handle watcher thread is joined.
-  // But that can't happen since the pump uses mojo message pipes to wake up the
-  // pump. Since mojo EDK has been shutdown already, this never completes.
-  return base::Singleton<WatcherThreadManager,
-                         base::LeakySingletonTraits<WatcherThreadManager>>::
-      get();
+  return base::Singleton<WatcherThreadManager>::get();
 }
 
 WatcherID WatcherThreadManager::StartWatching(