This new event will be consumed by DevTools to update style information.
As for `StyleSheetApplicableStateChanged`, this event is emitted from the document.
It's only enabled when `document.styleSheetChangeEventsEnabled` is set to true.
The existing test around the `StyleSheetApplicableStateChanged` is renamed and
modified to assert both events when stylesheets are added, modified and removed.
Differential Revision: https://phabricator.services.mozilla.com/D147271
Emilio Cobos Álvarez
Nicolas Chevobbe
Gijs Kruitbosch