Scale client area, hit test mask and bounds by device_scale_factor.

services/ui/demo/mus_demo.cc

Index: services/ui/demo/mus_demo.cc
diff --git a/services/ui/demo/mus_demo.cc b/services/ui/demo/mus_demo.cc
index 5f75e9aea083656f58c85522eda0cd0e0a959975..3b7f9e20ee8e79d0f65740ef57e1ec14a0fe678f 100644
--- a/services/ui/demo/mus_demo.cc
+++ b/services/ui/demo/mus_demo.cc
@@ -62,9 +62,13 @@ void DrawSquare(const gfx::Rect& bounds, double angle, SkCanvas* canvas) {
 
 MusDemo::MusDemo() {}
 
-MusDemo::~MusDemo() {}
+MusDemo::~MusDemo() {
+  display::Screen::SetScreenInstance(nullptr);
+}
 
 void MusDemo::OnStart() {
+  test_screen_.reset(new display::test::TestScreen());
+  display::Screen::SetScreenInstance(test_screen_.get());
   gpu_service_ = GpuService::Create(context()->connector());
   window_tree_client_ = base::MakeUnique<WindowTreeClient>(this, this);
   window_tree_client_->ConnectAsWindowManager(context()->connector());