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`.

libcef/browser/browser_host_base.cc

@@ -480,6 +480,10 @@ void CefBrowserHostBase::SetFocus(bool focus) {
   }
 }
 
+int CefBrowserHostBase::GetOpenerIdentifier() {
+  return opener_id_;
+}
+
 void CefBrowserHostBase::RunFileDialog(
     FileDialogMode mode,
     const CefString& title,
@@ -1445,6 +1449,11 @@ bool CefBrowserHostBase::IsVisible() const {
   return false;
 }
 
+int CefBrowserHostBase::GetNextPopupId() {
+  CEF_REQUIRE_UIT();
+  return next_popup_id_++;
+}
+
 bool CefBrowserHostBase::EnsureDevToolsProtocolManager() {
   CEF_REQUIRE_UIT();
   if (!contents_delegate_.web_contents()) {