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

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3310,8 +3310,12 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
               1. [=fetch controller/Abort=] |preloadFetchController| with |deserializedError|.
       1. Else, resolve |preloadResponse| with undefined.
       1. Let |fetchResult| be the result of [=Create Fetch Event and Dispatch=] with |request|, |registration|, |useHighResPerformanceTimers|, |timingInfo|, |workerRealm|, |reservedClient|, |preloadResponse|, and null.
-      1. If |fetchResult| is not null and |fetchResult|'s [=response/service worker timing info=]'s [=service worker timing info/worker final router source=] is set to {{RouterSourceEnum/"network"}}:
-        1. Set |fetchResult|'s [=response/service worker timing info=]'s [=service worker timing info/worker final router source=] to {{RouterSourceEnum/"fetch-event"}}.
+      1. If |timingInfo|'s [=service worker timing info/worker final router source=] is not an empty string:
+          1. Assert |timingInfo|'s [=service worker timing info/worker final router source=] is {{RouterSourceEnum/"network"}}.
+          1. If |fetchResult| is null, then return |timingInfo|.
+          1. Else:
+              1. Assert |fetchResult|'s [=response/service worker timing info=]'s [=service worker timing info/worker final router source=] is {{RouterSourceEnum/"network"}}.
+              1. Set |fetchResult|'s [=response/service worker timing info=]'s [=service worker timing info/worker final router source=] to {{RouterSourceEnum/"fetch-event"}}.
       1. Return |fetchResult|.
   </section>