Adds support for multiple displays to WmTestBase

services/ui/public/cpp/tests/window_tree_client_private.cc

Index: services/ui/public/cpp/tests/window_tree_client_private.cc
diff --git a/services/ui/public/cpp/tests/window_tree_client_private.cc b/services/ui/public/cpp/tests/window_tree_client_private.cc
index 514f980398fc11331407ea442b83bc311de7c0c1..77c802048f689841039408845b22ff4e6d141d81 100644
--- a/services/ui/public/cpp/tests/window_tree_client_private.cc
+++ b/services/ui/public/cpp/tests/window_tree_client_private.cc
@@ -23,7 +23,7 @@ WindowTreeClientPrivate::~WindowTreeClientPrivate() {}
 void WindowTreeClientPrivate::OnEmbed(mojom::WindowTree* window_tree) {
   mojom::WindowDataPtr root_data(mojom::WindowData::New());
   root_data->parent_id = 0;
-  root_data->window_id = 1;
+  root_data->window_id = next_window_id_++;
   root_data->properties.SetToEmpty();
   root_data->visible = true;
   const int64_t display_id = 1;
@@ -36,7 +36,7 @@ void WindowTreeClientPrivate::CallWmNewDisplayAdded(
     const display::Display& display) {
   mojom::WindowDataPtr root_data(mojom::WindowData::New());
   root_data->parent_id = 0;
-  root_data->window_id = 1;
+  root_data->window_id = next_window_id_++;
   root_data->properties.SetToEmpty();
   root_data->visible = true;
   root_data->bounds = gfx::Rect(display.bounds().size());