Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/host/chromoting_host_unittest.cc

Index: remoting/host/chromoting_host_unittest.cc
diff --git a/remoting/host/chromoting_host_unittest.cc b/remoting/host/chromoting_host_unittest.cc
index d37f79f2850f3593aa103e0b4e7289422344583f..a2fe0904de0a108ecd6a771628bf88ce47bc4a06 100644
--- a/remoting/host/chromoting_host_unittest.cc
+++ b/remoting/host/chromoting_host_unittest.cc
@@ -78,7 +78,7 @@ class ChromotingHostTest : public testing::Test {
     ON_CALL(context_, network_message_loop())
         .WillByDefault(Return(message_loop_proxy_.get()));
     ON_CALL(context_, ui_message_loop())
-        .WillByDefault(Return(&message_loop_));
+        .WillByDefault(Return(message_loop_proxy_.get()));
     EXPECT_CALL(context_, main_message_loop())
         .Times(AnyNumber());
     EXPECT_CALL(context_, encode_message_loop())
@@ -88,12 +88,6 @@ class ChromotingHostTest : public testing::Test {
     EXPECT_CALL(context_, ui_message_loop())
         .Times(AnyNumber());
 
-    context_.SetUITaskPostFunction(base::Bind(
-        static_cast<void(MessageLoop::*)(
-            const tracked_objects::Location&,
-            const base::Closure&)>(&MessageLoop::PostTask),
-        base::Unretained(&message_loop_)));
-
     Capturer* capturer = new CapturerFake();
     event_executor_ = new MockEventExecutor();
     curtain_ = new MockCurtain();