Makes WindowTree::SetFocus(null) work

components/mus/public/interfaces/window_tree.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mus.mojom;
 
 import "components/mus/public/interfaces/compositor_frame.mojom";
 import "components/mus/public/interfaces/cursor.mojom";
 import "components/mus/public/interfaces/input_events.mojom";
 import "components/mus/public/interfaces/mus_constants.mojom";
@@ skipping 197 matching lines @@
   // WindowTreeClient being removed. The embedded app is told this by way of
   // OnUnembed(), which is followed by OnWindowDeleted() (as the connection no
   // longer has access to the window).
   //
   // The embedder can detect when the embedded app disconnects by way of
   // OnEmbeddedAppDisconnected().
   //
   // The callback returns whether the embedding was successful.
   Embed(uint32 window_id, WindowTreeClient client) => (bool success);
 
+  // Sets focus to the specified window, use 0 to clear focus. For a window to
+  // get focus the following has to happen: the window is drawn, the window has
+  // been marked as focusable (see SetCanFocus()) and the window is in a
+  // container the WindowManager has identified as allowing activation
+  // (see WindowManagerClient::AddActivationParent()).
   SetFocus(uint32 change_id, uint32 window_id);
+
+  // Marks the specified window as being able to receive focus.
   SetCanFocus(uint32 window_id, bool can_focus);
 
   // Sets the cursor when the pointer is inside |window_id| to a system standard
   // cursor provided by the window manager.
   SetPredefinedCursor(uint32 change_id, uint32 window_id, Cursor cursor_id);
 
   // TODO(erg): Additional cursor methods. Image based cursors, visibility,
   // and cursor locking.
 
   // Set text input state for the given window.
@@ skipping 133 matching lines @@
   // See description of WindowManager for details.
   GetWindowManager(associated WindowManager& internal);
 };
 
 // Mus provides this interface as a way for clients to connect and obtain a
 // WindowTree handle with a supplied WindowTreeClient handle. The
 // WindowTreeClient has no roots, use NewTopLevelWindow() to create one.
 interface WindowTreeFactory {
   CreateWindowTree(WindowTree& tree_request, WindowTreeClient client);
 };