Add missing IWYU message_loop.h includes.

ppapi/proxy/ppapi_proxy_test.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/task_runner.h"
 #include "base/threading/simple_thread.h"
 #include "base/threading/thread.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/proxy/host_dispatcher.h"
 #include "ppapi/proxy/plugin_dispatcher.h"
 #include "ppapi/proxy/plugin_globals.h"
 #include "ppapi/proxy/plugin_proxy_delegate.h"
 #include "ppapi/proxy/plugin_resource_tracker.h"
@@ skipping 351 matching lines @@
 //   EXPECT_VAR_IS_STRING("foo", my_var);
 #define EXPECT_VAR_IS_STRING(str, var) { \
   StringVar* sv = StringVar::FromPPVar(var); \
   EXPECT_TRUE(sv); \
   if (sv) \
     EXPECT_EQ(str, sv->value()); \
 }
 
 }  // namespace proxy
 }  // namespace ppapi