Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/host/simple_host_process.cc

Index: remoting/host/simple_host_process.cc
diff --git a/remoting/host/simple_host_process.cc b/remoting/host/simple_host_process.cc
index 3e6000c01d17d39f54f2e82bc2304f29820dbb46..fb99e5228de38c61d17a6dac5ba44dfe3f125301 100644
--- a/remoting/host/simple_host_process.cc
+++ b/remoting/host/simple_host_process.cc
@@ -104,13 +104,8 @@ class SimpleHost {
     // It needs to be a UI message loop to keep runloops spinning on the Mac.
     MessageLoop message_loop(MessageLoop::TYPE_UI);
 
-    remoting::ChromotingHostContext context;
-    // static_cast needed to resolve overloaded PostTask member-function.
-    context.SetUITaskPostFunction(base::Bind(
-        static_cast<void(MessageLoop::*)(
-            const tracked_objects::Location&,
-            const base::Closure&)>(&MessageLoop::PostTask),
-        base::Unretained(&message_loop)));
+    remoting::ChromotingHostContext context(
+        base::MessageLoopProxy::CreateForCurrentThread());
     context.Start();
 
     base::Thread file_io_thread("FileIO");