aura-mus: Implement Deactivate().

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index 5946488013fd18246e613b9e236117fec04072cf..8c815b69a95d8e0c752184dfbc47a1f77005bc8c 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -499,6 +499,17 @@ void WindowManager::OnWmSetClientArea(
   non_client_frame_controller->SetClientArea(insets, additional_client_areas);
 }
 
+void WindowManager::OnWmDeactivateWindow(aura::Window* window) {
+  aura::client::ActivationClient* activation_client =
+      static_cast<aura::client::ActivationClient*>(focus_controller_.get());
+  if (activation_client->GetActiveWindow() != window) {
+    DVLOG(1) << "Non-active window requested deactivation.";
+    return;
+  }
+
+  activation_client->DeactivateWindow(window);
+}
+
 void WindowManager::OnWindowInitialized(aura::Window* window) {
   // This ensures WmWindowAura won't be called before WmWindowMus. This is
   // important as if WmWindowAura::Get() is called first, then WmWindowAura