Sample that shows how to use the context menu API in an app to achieve per-window menus.
Context menus are normally global to the entire app, and thus all windows would have the same menu. This sample uses a
focus event handler in each window to detect when a window is brought to the foreground. When it is, the contents of the context menu are updated with that window's commands. The
chrome.contextMenus.onClicked event handler also only handles events that occur in that window.