Fix race on mojo message pump shutdown.

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 dbc20eb7992d11a12ab7f2ff6209b0e4b9a63d59..97d6d8856606d732607c2af3c38ae97b144c01b4 100644
--- a/mojo/message_pump/handle_watcher.cc
+++ b/mojo/message_pump/handle_watcher.cc
@@ -275,11 +275,6 @@ void WatcherThreadManager::AddRequest(const RequestData& data) {
       return;
   }
 
-  // With the new Mojo EDK, the MojoMessagePump will destruct when it sees that
-  // the IO thread has gone away. So at shutdown this can be null.
-  if (!thread_.task_runner())
-    return;
-
   // We outlive |thread_|, so it's safe to use Unretained() here.
   thread_.task_runner()->PostTask(
       FROM_HERE,