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..ef3d594b3f4deb15d86deda14108b5518282f7b6 100644
--- a/mojo/message_pump/handle_watcher.cc
+++ b/mojo/message_pump/handle_watcher.cc
@@ -222,7 +222,9 @@ WatcherThreadManager::~WatcherThreadManager() {
 }
 
 WatcherThreadManager* WatcherThreadManager::GetInstance() {
-  return base::Singleton<WatcherThreadManager>::get();
+  return base::Singleton<WatcherThreadManager,
+                         base::LeakySingletonTraits<WatcherThreadManager>>::

[jam, 2015/11/07 05:00:31]

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.

[sky, 2015/11/08 16:26:36]

Please add this comment to the code here.

[jam, 2015/11/09 18:20:58]

Done.

+      get();
 }
 
 WatcherID WatcherThreadManager::StartWatching(