Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/host/chromoting_host_context_unittest.cc

 // Copyright (c) 2011 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 "base/message_loop.h"
+#include "base/message_loop_proxy.h"
 #include "remoting/host/chromoting_host_context.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace remoting {
 
 // A simple test that starts and stop the context. This tests the context
 // operates properly and all threads and message loops are valid.
 TEST(ChromotingHostContextTest, StartAndStop) {
-  ChromotingHostContext context;
+  MessageLoop message_loop;
+  ChromotingHostContext context(
+      base::MessageLoopProxy::CreateForCurrentThread());
+
   context.Start();
   EXPECT_TRUE(context.jingle_thread());
   EXPECT_TRUE(context.main_message_loop());
   EXPECT_TRUE(context.encode_message_loop());
   context.Stop();
 
   // Expect all the threads are stopped.
   EXPECT_FALSE(context.main_thread_.IsRunning());
   EXPECT_FALSE(context.encode_thread_.IsRunning());
 }
 
 }  // namespace remoting