Scale bounds in aura-mus client-lib and NativeWidgetMus in hdpi mode.

ui/aura/mus/window_tree_host_mus.cc

Index: ui/aura/mus/window_tree_host_mus.cc
diff --git a/ui/aura/mus/window_tree_host_mus.cc b/ui/aura/mus/window_tree_host_mus.cc
index c5c314b025a28991f753584410aefd0d430d1126..ac0aa419a73f14cc4db624087ef201412ba223e4 100644
--- a/ui/aura/mus/window_tree_host_mus.cc
+++ b/ui/aura/mus/window_tree_host_mus.cc
@@ -65,8 +65,10 @@ WindowTreeHostMus::WindowTreeHostMus(
         static_cast<gfx::AcceleratedWidget>(accelerated_widget_count++);
 #endif
   }
-  // TODO(markdittmer): Use correct device-scale-factor from |window|.
-  OnAcceleratedWidgetAvailable(accelerated_widget, 1.f);
+  OnAcceleratedWidgetAvailable(accelerated_widget,
+                               display::Screen::GetScreen()
+                                   ->GetDisplayNearestWindow(window())
+                                   .device_scale_factor());
 
   delegate_->OnWindowTreeHostCreated(this);