Bug 1501899 - Label the Identity popup with the Site information header for accessibility, r=johannh

Differential Revision: https://phabricator.services.mozilla.com/D9757

--HG--
extra : moz-landing-system : lando

browser/components/controlcenter/content/panel.inc.xul

@@ -7,6 +7,7 @@
        hidden="true"
        photon="true"
        role="alertdialog"
+       aria-labelledby="identity-popup-mainView-panel-header-span"
        onpopupshown="gIdentityHandler.onPopupShown(event);"
        onpopuphidden="gIdentityHandler.onPopupHidden(event);"
        orient="vertical">