Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/host/host_mock_objects.cc

Index: remoting/host/host_mock_objects.cc
diff --git a/remoting/host/host_mock_objects.cc b/remoting/host/host_mock_objects.cc
index de0ea970e684c85615d5fafb3e626c32aaea4cce..168e3e8f91bc71ac140359744032c996b70a319d 100644
--- a/remoting/host/host_mock_objects.cc
+++ b/remoting/host/host_mock_objects.cc
@@ -4,6 +4,8 @@
 
 #include "remoting/host/host_mock_objects.h"
 
+#include "base/message_loop_proxy.h"
+
 namespace remoting {
 
 MockCapturer::MockCapturer() {}
@@ -46,7 +48,9 @@ LocalInputMonitor* LocalInputMonitor::Create() {
   return new MockLocalInputMonitor();
 }
 
-MockChromotingHostContext::MockChromotingHostContext() {}
+MockChromotingHostContext::MockChromotingHostContext()
+    : ChromotingHostContext(base::MessageLoopProxy::CreateForCurrentThread()) {
+}
 
 MockChromotingHostContext::~MockChromotingHostContext() {}