Makes WindowTree::SetFocus(null) work

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index 2583e912ee9a25cf7889c685b1e3109da724df5c..90bdec063e93ff7f2d30251ad874af8e118b956b 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -215,7 +215,14 @@ interface WindowTree {
   // 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