Use ScopedTaskEnvironment instead of MessageLoop in mojo unit tests.

mojo/edk/system/ports/ports_unittest.cc

Index: mojo/edk/system/ports/ports_unittest.cc
diff --git a/mojo/edk/system/ports/ports_unittest.cc b/mojo/edk/system/ports/ports_unittest.cc
index cb48b3eb2f4100ac901db4fead045c2591ba8f9d..4b384b49f677886c7268bbee4db1943f2da8629d 100644
--- a/mojo/edk/system/ports/ports_unittest.cc
+++ b/mojo/edk/system/ports/ports_unittest.cc
@@ -21,6 +21,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/lock.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread.h"
 #include "mojo/edk/system/ports/event.h"
 #include "mojo/edk/system/ports/node.h"
@@ -427,7 +428,7 @@ class PortsTest : public testing::Test, public MessageRouter {
     }
   }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   // Acquired before any operation which makes a Node busy, and before testing
   // if all nodes are idle.