Base structure.
Called whenever draggable regions for the browser window change. These can be specified using the '-webkit-app-region: drag/no-drag' CSS-property. If draggable regions are never defined in a document this function will also never be called. If the last draggable region is removed from a document this function will be called with an NULL vector.
Implement this structure to handle events related to dragging. The functions of this structure will be called on the UI thread.