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_win.h

@@ -126,7 +126,6 @@ class RootWindowWin : public RootWindow, public BrowserWindow::Delegate {
   cef_show_state_t initial_show_state_ = CEF_SHOW_STATE_NORMAL;
   std::unique_ptr<BrowserWindow> browser_window_;
   CefBrowserSettings browser_settings_;
-  bool initialized_ = false;
 
   // Main window.
   HWND hwnd_ = nullptr;