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

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 97c490aff4ee5ef59183176725cbeb95ae0bdabb..80b3fa015bcc3ebb07bac80d743a14b2c59cf6ae 100644
--- a/remoting/host/chromoting_host_unittest.cc
+++ b/remoting/host/chromoting_host_unittest.cc
@@ -294,6 +294,10 @@ class ChromotingHostTest : public testing::Test {
     EXPECT_CALL(*desktop_environment, CreateEventExecutorPtr(_, _))
         .Times(AnyNumber())
         .WillRepeatedly(Invoke(this, &ChromotingHostTest::CreateEventExecutor));
+    EXPECT_CALL(*desktop_environment, CreateSessionControllerPtr())
+        .Times(AnyNumber())
+        .WillRepeatedly(Invoke(this,
+                               &ChromotingHostTest::CreateSessionController));
     EXPECT_CALL(*desktop_environment, CreateVideoCapturerPtr(_, _))
         .Times(AnyNumber())
         .WillRepeatedly(Invoke(this, &ChromotingHostTest::CreateVideoCapturer));
@@ -311,6 +315,12 @@ class ChromotingHostTest : public testing::Test {
     return event_executor;
   }
 
+  // Creates a dummy SessionController, to mock
+  // DesktopEnvironment::CreateSessionController().
+  SessionController* CreateSessionController() {
+    return new MockSessionController();
+  }
+
   // Creates a fake media::ScreenCapturer, to mock
   // DesktopEnvironment::CreateVideoCapturer().
   media::ScreenCapturer* CreateVideoCapturer(