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.cc

@@ -1017,6 +1017,8 @@ void RootWindowWin::OnCreate(LPCREATESTRUCT lpCreateStruct) {
     browser_window_->ShowPopup(hwnd_, rect.left, rect.top,
                                rect.right - rect.left, rect.bottom - rect.top);
   }
+
+  window_created_ = true;
 }
 
 bool RootWindowWin::OnClose() {