Less fragile |fetchController| termination check (w3c#1780)

https://infra.spec.whatwg.org/#abort-when adds the check each time the step is executed. However, there is only a step
between and the abort-when and if-aborted, it should be less effective abort than we expect. Let me add a step inside
the callback in case.
This is the fix for that.

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3263,15 +3263,17 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
                   1. Let |queue| be an empty [=queue=] of [=race result=].
                   1. Let |raceFetchController| be null.
                   1. Let |raceResponse| be a [=race response=] whose [=race response/value=] is "<code>pending</code>".
-                  1. Run the following substeps [=in parallel=], but [=abort when=] |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>":
+                  1. Run the following substeps [=in parallel=]:
+                      1. If |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>", then set |raceResponse| to a [=race response=] whose [=race response/value=] is null, and abort these steps.
                       1. Set |raceFetchController| to the result of calling [=fetch=] given |request|, with [=fetch/processResponse=] set to the following steps given a [=/response=] |raceNetworkRequestResponse|:
-                          1. Set |raceResponse|'s [=race response/value=] to |raceNetworkRequestResponse|.
-                          1. If |raceNetworkRequestResponse|'s [=response/status=] is [=ok status=], then:
-                              1. Let |raceNetworkResult| be a [=race result=] whose [=race result/routed response=] is |raceNetworkRequestResponse| and [=race result/used route=] is {{RouterSourceEnum/"network"}}.
-                              1. [=queue/Enqueue=] |raceNetworkResult| to |queue|.
-                  1. [=If aborted=] and |raceFetchController| is not null, then:
-                      1. [=fetch controller/Abort=] |raceFetchController|.
-                      1. Set |raceResponse| to a [=race response=] whose [=race response/value=] is null.
+                          1. Run these steps, but [=abort when=] |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>".
+                              1. Set |raceResponse|'s [=race response/value=] to |raceNetworkRequestResponse|.
+                              1. If |raceNetworkRequestResponse|'s [=response/status=] is [=ok status=], then:
+                                  1. Let |raceNetworkResult| be a [=race result=] whose [=race result/routed response=] is |raceNetworkRequestResponse| and [=race result/used route=] is {{RouterSourceEnum/"network"}}.
+                                  1. [=queue/Enqueue=] |raceNetworkResult| to |queue|.
+                          1. [=If aborted=] and |raceFetchController| is not null, then:
+                              1. [=fetch controller/Abort=] |raceFetchController|.
+                              1. Set |raceResponse| to a [=race response=] whose [=race response/value=] is null.
                   1. Resolve |preloadResponse| with undefined.
                   1. Run the following substeps [=in parallel=]:
                       1. Let |fetchHandlerResponse| be the result of [=Create Fetch Event and Dispatch=] with |request|, |registration|, |useHighResPerformanceTimers|, |timingInfo|, |workerRealm|, |reservedClient|, |preloadResponse|, and |raceResponse|.