mus/views: Fix creating gfx::Display in high-dpi.

ui/aura/test/aura_test_helper.cc

Index: ui/aura/test/aura_test_helper.cc
diff --git a/ui/aura/test/aura_test_helper.cc b/ui/aura/test/aura_test_helper.cc
index 2d679e59f184c7f2248e374f1e65832776bb6b06..10c45ca333b9ed6be2460f55055a3180c0b51db4 100644
--- a/ui/aura/test/aura_test_helper.cc
+++ b/ui/aura/test/aura_test_helper.cc
@@ -67,9 +67,12 @@ void AuraTestHelper::SetUp(ui::ContextFactory* context_factory) {
 
   ui::InitializeInputMethodForTesting();
 
-  gfx::Size host_size(800, 600);
+  gfx::Screen* screen = gfx::Screen::GetScreen();
+  gfx::Size host_size(screen ? screen->GetPrimaryDisplay().GetSizeInPixel()
+                             : gfx::Size(800, 600));
   test_screen_.reset(TestScreen::Create(host_size));
-  gfx::Screen::SetScreenInstance(test_screen_.get());
+  if (!screen)
+    gfx::Screen::SetScreenInstance(test_screen_.get());
   host_.reset(test_screen_->CreateHostForPrimaryDisplay());
 
   focus_client_.reset(new TestFocusClient);