Issue #2472 - Part 3: Refactor Fullscreen stack to Top Layer stack
    
    Fullscreen stack isn't part of the spec anymore; it's been changed to a more
    generic version called Top Layer stack, which is being used by both full-screen
    APIs and dialog elements.
    
    This patch refactors it to Top Layer stack so that it can be reused for dialog
    elements.
    
    Top Layer stack spec: https://fullscreen.spec.whatwg.org/#new-stacking-layer