Fix: Ensure timingInfo is returned on network fallback if SW static routing API is used (#1774)
According to the explainer, the worker final router source should be 'network' for a network fallback. The |timingInfo|
is returned to make that happen.
https://github.com/WICG/service-worker-static-routing-api/blob/main/resource-timing-api.md#correspondence-of-the-matched-source-type-and-the-actual-source-type