Support creation of toplevel mus::Windows on separate displays

ash/mus/root_window_controller_unittest.cc

Index: ash/mus/root_window_controller_unittest.cc
diff --git a/ash/mus/root_window_controller_unittest.cc b/ash/mus/root_window_controller_unittest.cc
index 7053e14cb3512b256d0bc44e155230cb3f1d6292..912e2d7494f6958fc6e11e14bc130995c0539bd0 100644
--- a/ash/mus/root_window_controller_unittest.cc
+++ b/ash/mus/root_window_controller_unittest.cc
@@ -7,6 +7,7 @@
 #include "ash/common/test/ash_test.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
+#include "ash/mus/test/wm_test_base.h"
 
 namespace ash {
 
@@ -20,4 +21,25 @@ TEST_F(RootWindowControllerTest, CreateFullscreenWindow) {
   EXPECT_EQ(root_window->GetBounds(), window->GetBounds());
 }
 
+using RootWindowControllerWmTest = mus::WmTestBase;
+
+TEST_F(RootWindowControllerWmTest, IsWindowShownInCorrectDisplay) {
+  if (!SupportsMultipleDisplays())
+    return;
+
+  UpdateDisplay("400x400,400x400");
+  EXPECT_NE(GetPrimaryDisplay().id(), GetSecondaryDisplay().id());
+
+  ui::Window* window_primary_display =
+      CreateFullscreenTestWindow(GetPrimaryDisplay().id());
+  ui::Window* window_secondary_display =
+      CreateFullscreenTestWindow(GetSecondaryDisplay().id());
+
+  DCHECK(window_primary_display);

[sky, 2016/11/29 17:43:13]

For tests prefer ASSERT over DCHECK. The reason is in a test you want other
tests to be run if one fails. A DCHECK will cause a crash and no other tests to
run.

+  DCHECK(window_secondary_display);
+
+  EXPECT_EQ(window_primary_display->display_id(), GetPrimaryDisplay().id());

[sky, 2016/11/29 17:43:13]

In chrome code we use the format of expected, actual. So, the args here should
be reversed.

+  EXPECT_EQ(window_secondary_display->display_id(), GetSecondaryDisplay().id());
+}
+
 }  // namespace ash