Set request's initiator and destination to empty string in cache.addAll()

As pointed out in #1718, "fetch"
is not a valid value for a request's initiator, and "subresource" is not
a valid destination in the Fetch specification.

Following the discussion in the issue, cache.addAll() should function
the same as if a script ran fetch() directly. Therefore, both initiator
and destination should be set to the empty string, which is an initial value.
i.e. removed the initialization.

Fixes #1718

Author: yoshisatoyanagisawa

index.bs

@@ -2099,7 +2099,6 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
                 1. [=list/For each=] |fetchController| of |fetchControllers|, [=fetch controller/abort=] |fetchController|.
                 1. Return [=a promise rejected with=] a `TypeError`.
             1. If |r|'s [=request/client=]'s [=environment settings object/global object=] is a {{ServiceWorkerGlobalScope}} object, set |request|'s [=service-workers mode=] to "`none`".
-            1. Set |r|'s [=request/initiator=] to "`fetch`" and [=request/destination=] to "`subresource`".
             1. Add |r| to |requestList|.
             1. Let |responsePromise| be [=a new promise=].
             1. Run the following substeps [=in parallel=]: