Add notification for aborted popups (fixes #3776)

Pass a new |popup_id| parameter to OnBeforePopup and call a new
OnBeforePopupAborted callback if the popup is aborted before
OnAfterCreated is called for the popup browser. Add new
CefBrowserHost::GetBrowserByIdentifier and GetOpenerIdentifier
methods to assist with retrieval of associated browsers.

In cefclient, clean up state when a popup is aborted and close
any associated popup browsers when the opener browser is closed.
This also works when running with `--use-default-popup`.

tests/cefclient/browser/root_window_views.h

@@ -105,7 +105,6 @@ class RootWindowViews : public RootWindow,
   // Members set during initialization. Safe to access from any thread.
   std::unique_ptr<RootWindowConfig> config_;
   CefRefPtr<ClientHandler> client_handler_;
-  bool initialized_ = false;
 
   // Only accessed on the main thread.
   CefRefPtr<CefBrowser> browser_;