aura-mus: Implement Deactivate().

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index b6c804ffd51237936eb2bb25191cf82b8579d515..7990e305abb650d2443595adddd99abc825b2eea 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -260,6 +260,10 @@ interface WindowTree {
   // See documentation for WindowTreeClient::OnWindowInputEvent().
   OnWindowInputEventAck(uint32 event_id, EventResult result);
 
+  // If the current focus is (or is a child of) |window_id|, requests that the
+  // window manager change the focus to the next activatable window.
+  DeactivateWindow(uint32 window_id);
+
   // See description of WindowManager for details.
   GetWindowManagerClient(associated WindowManagerClient& internal);