Mojo: Fix DCHECK on middle-click in window manager.

TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/333173003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278813 0039d316-1c4b-4281-b951-d872f2087c98

mojo/examples/window_manager/window_manager.cc

@@ -116,7 +116,8 @@ class WindowManager : public Application,
         app_url = "mojo://mojo_embedded_app";
       else if (event->flags & ui::EF_RIGHT_MOUSE_BUTTON)
         app_url = "mojo://mojo_nesting_app";
-      DCHECK(!app_url.empty());
+      if (app_url.empty())
+        return;
 
       Node* node = view_manager_->GetNodeById(parent_node_id_);
       navigation::NavigationDetailsPtr nav_details(