Cleanup display creation in mus tests.

services/ui/ws/platform_display_default.h

Index: services/ui/ws/platform_display_default.h
diff --git a/services/ui/ws/platform_display_default.h b/services/ui/ws/platform_display_default.h
index 215a9f0ec83c3a0ac715b915619571c98eca2ead..2b560c6a0fa28928c58ebf3ceffd20420f627b86 100644
--- a/services/ui/ws/platform_display_default.h
+++ b/services/ui/ws/platform_display_default.h
@@ -44,7 +44,6 @@ class PlatformDisplayDefault : public PlatformDisplay,
   gfx::Rect GetBounds() const override;
   bool UpdateViewportMetrics(const display::ViewportMetrics& metrics) override;
   const display::ViewportMetrics& GetViewportMetrics() const override;
-  bool IsPrimaryDisplay() const override;
   void OnGpuChannelEstablished(
       scoped_refptr<gpu::GpuChannelHost> gpu_channel) override;
 
@@ -72,7 +71,7 @@ class PlatformDisplayDefault : public PlatformDisplay,
   // FrameGeneratorDelegate:
   bool IsInHighContrastMode() override;
 
-  int64_t display_id_;
+  const int64_t display_id_;
 
 #if !defined(OS_ANDROID)
   std::unique_ptr<ui::CursorLoader> cursor_loader_;