Called when web content in the page has toggled fullscreen mode. If |fullscreen| is true (1) the content will automatically be sized to fill the browser content area. If |fullscreen| is false (0) the content will automatically return to its original size and position. With the Alloy runtime the client is responsible for triggering the fullscreen transition (for example, by calling cef_window_t::SetFullscreen when using Views). With the Chrome runtime the fullscreen transition will be triggered automatically. The cef_window_delegate_t::OnWindowFullscreenTransition function will be called during the fullscreen transition for notification purposes.