Removes WebContentsViewMus/RenderWidgetHostViewMus

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 6f6c35090eac6685e39216ac8f3fa86a5eb27309..10a25a3a5f241c711602d5fb4d7c58674d261782 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -264,6 +264,9 @@ void WindowTreeClient::Embed(
     uint32_t flags,
     const ui::mojom::WindowTree::EmbedCallback& callback) {
   DCHECK(tree_);
+  // Window::Init() must be called before Embed() (otherwise the server hasn't
+  // been told about the window).
+  DCHECK(window->layer());
   if (!window->children().empty()) {
     // The window server removes all children before embedding. In other words,
     // it's generally an error to Embed() with existing children. So, fail