imgRequestProxy::IsOnEventTarget must return false in order for imgRequestProxy::Dispatch to be called. Typically we check for mListener before any of this but in imgRequest::OnLoadComplete, we have other things to do besides notifying the listener. As such, we want to dispatch even if there is no listener, and that is when the assert can fail. Since IsOnEventTarget can only return false if it has either a tab group *or* a listener, we can change the assert to match.
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