Converts WindowTreeClientTest to be in terms of aura

ui/aura/mus/window_tree_client_delegate.h

Index: ui/aura/mus/window_tree_client_delegate.h
diff --git a/ui/aura/mus/window_tree_client_delegate.h b/ui/aura/mus/window_tree_client_delegate.h
index dca30464266a448ba141af5f116f0c9285f84165..609a05785854c6fd8accfccc5d02dfdd67f333df 100644
--- a/ui/aura/mus/window_tree_client_delegate.h
+++ b/ui/aura/mus/window_tree_client_delegate.h
@@ -38,10 +38,17 @@ class AURA_EXPORT WindowTreeClientDelegate {
   virtual void OnUnembed(Window* root);
 
   // Called when the embed root has been destroyed on the server side (or
-  // another client was embedded in the same window). This is only called if the
-  // WindowTreeClient was created from an InterfaceRequest. Generally when this
-  // is called the delegate should delete the WindowTreeClient.
-  virtual void OnEmbedRootDestroyed(Window* root) = 0;
+  // another client was embedded in the same window). This is called in two
+  // distinct cases:
+  // . If the WindowTreeClient was created from an InterfaceRequest. In this
+  //   case the WindowTreeClient associated with the WindowTreeHost is no
+  //   longer in a usable state and should be deleted (after deleting
+  //   |window_tree_host|).
+  // . When the window associated with a top level window (one created by way of
+  //   directly creating a WindowTreeHostMus) is destroyed on the server side.
+  //   In this case the |window_tree_host| should be destroyed, but the
+  //   WindowTreeClient is still usable.
+  virtual void OnEmbedRootDestroyed(WindowTreeHostMus* window_tree_host) = 0;
 
   // Called when the connection to the window server has been lost. After this
   // call the windows are still valid, and you can still do things, but they