Now that we have support for timeline panel in workers we need to make sure that InspectorController::hasFrontends() works for workers as well. The problem here is that now frontend counter is a static variable while we need a thread local one. Turning it into a thread-local may have significant performance impact in some cases so wee have to be careful.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.