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 964f1e2d3348d80b1edef376538ce2eac0862639..1397dee9cc2290a9252acb95a88c570cf8e9b36b 100644
--- a/mojo/message_pump/handle_watcher.cc
+++ b/mojo/message_pump/handle_watcher.cc
@@ -222,7 +222,13 @@ WatcherThreadManager::~WatcherThreadManager() {
 }
 
 WatcherThreadManager* WatcherThreadManager::GetInstance() {
-  return base::Singleton<WatcherThreadManager>::get();
+  // 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();
 }
 
 WatcherID WatcherThreadManager::StartWatching(