mus: Allow the WM to specify the windows that can have active children.

mandoline/ui/desktop_ui/browser_window.cc

Index: mandoline/ui/desktop_ui/browser_window.cc
diff --git a/mandoline/ui/desktop_ui/browser_window.cc b/mandoline/ui/desktop_ui/browser_window.cc
index 067bfb6687363bddf9fbe93bc701d80f48e51bcf..0de2dbc4cd8df7961712c61952465b05095f0136 100644
--- a/mandoline/ui/desktop_ui/browser_window.cc
+++ b/mandoline/ui/desktop_ui/browser_window.cc
@@ -202,6 +202,7 @@ void BrowserWindow::OnEmbed(mus::Window* root) {
   host_->SetSize(mojo::Size::From(gfx::Size(1280, 800)));
 
   root_->AddChild(content_);
+  host_->SetActivationParent(root_->id(), true);
   content_->SetVisible(true);
 
   web_view_.Init(app_, content_);