Adds DesktopWindowTreeHostMus

ui/views/test/platform_test_helper.h

Index: ui/views/test/platform_test_helper.h
diff --git a/ui/views/test/platform_test_helper.h b/ui/views/test/platform_test_helper.h
index 236446afac0b10aa93313824b918fc667f7c9b2f..ada95a28cf07a460a8c62d68a94a45df0ddc07ed 100644
--- a/ui/views/test/platform_test_helper.h
+++ b/ui/views/test/platform_test_helper.h
@@ -12,6 +12,8 @@
 
 namespace views {
 
+class ViewsTestHelper;
+
 class PlatformTestHelper {
  public:
   using Factory = base::Callback<std::unique_ptr<PlatformTestHelper>(void)>;
@@ -26,6 +28,10 @@ class PlatformTestHelper {
   static void SetIsMus();
   static bool IsMus();
 
+  // Called once the ViewsTestHelper has been created, but before SetUp() is
+  // called.
+  virtual void OnTestHelperCreated(ViewsTestHelper* helper) {}
+
  private:
   DISALLOW_COPY_AND_ASSIGN(PlatformTestHelper);
 };