Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/host/chromoting_host_context_unittest.cc

Index: remoting/host/chromoting_host_context_unittest.cc
diff --git a/remoting/host/chromoting_host_context_unittest.cc b/remoting/host/chromoting_host_context_unittest.cc
index 21bcab8dcbb68124bdadcd67329aeda725ee8c00..3d857b3302f6eb382ada3c88997bd3433b2e153b 100644
--- a/remoting/host/chromoting_host_context_unittest.cc
+++ b/remoting/host/chromoting_host_context_unittest.cc
@@ -3,6 +3,7 @@
 // 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"
 
@@ -11,7 +12,10 @@ 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::current());
+
   context.Start();
   EXPECT_TRUE(context.jingle_thread());
   EXPECT_TRUE(context.main_message_loop());