ResizingHostObserver is created by the desktop environment together with other stubs.

remoting/host/client_session_unittest.cc

Index: remoting/host/client_session_unittest.cc
diff --git a/remoting/host/client_session_unittest.cc b/remoting/host/client_session_unittest.cc
index 9eb6348c5315d0d2a99fe170d4979f84e8490fc2..a7893a418d75b8fbec21b03e8d910842bcac856c 100644
--- a/remoting/host/client_session_unittest.cc
+++ b/remoting/host/client_session_unittest.cc
@@ -77,6 +77,10 @@ class ClientSessionTest : public testing::Test {
       scoped_refptr<base::SingleThreadTaskRunner> input_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
 
+  // Creates a dummy SessionController, to mock
+  // DesktopEnvironment::CreateSessionController().
+  SessionController* CreateSessionController();
+
   // Creates a fake media::ScreenCapturer, to mock
   // DesktopEnvironment::CreateVideoCapturer().
   media::ScreenCapturer* CreateVideoCapturer(
@@ -195,6 +199,8 @@ DesktopEnvironment* ClientSessionTest::CreateDesktopEnvironment() {
       .Times(0);
   EXPECT_CALL(*desktop_environment, CreateEventExecutorPtr(_, _))
       .WillOnce(Invoke(this, &ClientSessionTest::CreateEventExecutor));
+  EXPECT_CALL(*desktop_environment, CreateSessionControllerPtr())
+      .WillOnce(Invoke(this, &ClientSessionTest::CreateSessionController));
   EXPECT_CALL(*desktop_environment, CreateVideoCapturerPtr(_, _))
       .WillOnce(Invoke(this, &ClientSessionTest::CreateVideoCapturer));
 
@@ -208,6 +214,10 @@ EventExecutor* ClientSessionTest::CreateEventExecutor(
   return event_executor_.release();
 }
 
+SessionController* ClientSessionTest::CreateSessionController() {
+  return new MockSessionController();
+}
+
 media::ScreenCapturer* ClientSessionTest::CreateVideoCapturer(
     scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) {