Inform window manager about modal windows in mus+ash.

services/ui/public/interfaces/window_manager.mojom

Index: services/ui/public/interfaces/window_manager.mojom
diff --git a/services/ui/public/interfaces/window_manager.mojom b/services/ui/public/interfaces/window_manager.mojom
index 6ebca32648a3b68b116a809190dcc9b64e2f271e..b12905834ffe0eef6781c2dc1de5222c3203e557 100644
--- a/services/ui/public/interfaces/window_manager.mojom
+++ b/services/ui/public/interfaces/window_manager.mojom
@@ -8,6 +8,7 @@ import "services/ui/public/interfaces/cursor.mojom";
 import "services/ui/public/interfaces/event_matcher.mojom";
 import "services/ui/public/interfaces/window_manager_constants.mojom";
 import "services/ui/public/interfaces/window_tree_constants.mojom";
+import "ui/base/mojo/ui_base_types.mojom";
 import "ui/display/mojo/display.mojom";
 import "ui/events/mojo/event.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
@@ -170,6 +171,7 @@ interface WindowManager {
                 string name,
                 array<uint8>? value);
 
+  WmSetModalType(uint32 window_id, ModalType type);
   WmSetCanFocus(uint32 window_id, bool can_focus);
 
   // Asks the WindowManager to create a new window.