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/chrome/chrome_browser_host_impl.cc

@@ -500,6 +500,8 @@ void ChromeBrowserHostImpl::Attach(content::WebContents* web_contents,
   DCHECK(web_contents);
 
   if (opener) {
+    opener_id_ = opener->GetIdentifier();
+
     // Give the opener browser's platform delegate an opportunity to modify the
     // new browser's platform delegate.
     opener->platform_delegate()->PopupWebContentsCreated(