Support creation of toplevel mus::Windows on separate displays

ash/mus/test/wm_test_base.cc

Index: ash/mus/test/wm_test_base.cc
diff --git a/ash/mus/test/wm_test_base.cc b/ash/mus/test/wm_test_base.cc
index c45eb6e73457fd27f8e573754940c7f90f90f533..c480927f3b3340e79d4a624b6360dacabfab59ab 100644
--- a/ash/mus/test/wm_test_base.cc
+++ b/ash/mus/test/wm_test_base.cc
@@ -14,7 +14,6 @@
 #include "ash/mus/window_manager_application.h"
 #include "services/ui/public/cpp/property_type_converters.h"
 #include "services/ui/public/cpp/window_tree_client.h"
-#include "ui/display/display.h"
 
 namespace ash {
 namespace mus {
@@ -123,11 +122,15 @@ ui::Window* WmTestBase::CreateTestWindow(const gfx::Rect& bounds,
   return window;
 }
 
-ui::Window* WmTestBase::CreateFullscreenTestWindow() {
+ui::Window* WmTestBase::CreateFullscreenTestWindow(int64_t display_id) {
   std::map<std::string, std::vector<uint8_t>> properties;
   properties[ui::mojom::WindowManager::kShowState_Property] =
       mojo::ConvertTo<std::vector<uint8_t>>(
           static_cast<int32_t>(ui::mojom::ShowState::FULLSCREEN));
+
+  properties[ui::mojom::WindowManager::kInitialDisplayId_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(display_id);

[kylechar, 2016/11/23 00:28:00]

For the default value kInvalidDisplayId, do you really want to insert the
property with a value of -1? Or do you just want to not insert the property in
that case?

[thanhph, 2016/11/23 01:05:02]

Good question. I found a use case of kInvalidDisplayId here 
https://cs.chromium.org/chromium/src/ash/mus/property_util.cc?sq=package:chro...

which I interpret as we shouldn't add the property with a value of -1 since the
function GetInitialDisplayId will return -1 in this case. Since uint8_t is used,
I also assume display_id must be non negative, otherwise default display is used
to show window. If not correct, let me know. Thanks!

[kylechar, 2016/11/23 14:14:20]

So the value kInvalidDisplayId is appropriate to use to represent the concept of
"no display specified", since it's a reserved value that will never correspond
to a display. That part is fine.

I meant that if no display is specified then maybe it's better to not add that
property to the properties map, since it's meaningless. The value needs to be
serialized, deserialized, converted back to int64_t and then dropped (it's a lot
of work for nothing).

The last part about the value of |display_id| having to be non-negative is
wrong. |display_id| is int64_t or 8 bytes. It's being converted to a
std::vector<uint8_t>, not uint8_t, and I'd imagine you'll find that the vector
length is 8. All properties are just encoded into a vector of bytes (which are
represented by uint8_t).

+
   ui::Window* window = test_helper_->GetRootsOrderedByDisplayId()[0]
                            ->window_manager()
                            ->NewTopLevelWindow(&properties);