Adds DesktopWindowTreeHostMus

ui/views/test/views_test_helper_aura.h

Index: ui/views/test/views_test_helper_aura.h
diff --git a/ui/views/test/views_test_helper_aura.h b/ui/views/test/views_test_helper_aura.h
index 5acc7ec04356f89346b03141cc065783d2ad8cca..8ec540f90bd92ce52335ed43423a826710cba5a9 100644
--- a/ui/views/test/views_test_helper_aura.h
+++ b/ui/views/test/views_test_helper_aura.h
@@ -12,6 +12,8 @@
 #include "ui/views/test/views_test_helper.h"
 
 namespace aura {
+class WindowTreeClient;
+
 namespace client {
 class ScreenPositionClient;
 }
@@ -32,6 +34,9 @@ class ViewsTestHelperAura : public ViewsTestHelper {
                       ui::ContextFactory* context_factory);
   ~ViewsTestHelperAura() override;
 
+  void EnableMusWithWindowTreeClient(
+      aura::WindowTreeClient* window_tree_client);
+
   // Overridden from ViewsTestHelper:
   void SetUp() override;
   void TearDown() override;