Fix some declarative net request test expectations with Network Service

Because the WebRequest API stack doesn't see network requests at all
when no listeners are installed and the Network Service is enabled, this
adjusts a few test expectations which are invalid in the Network Service
path.

Bug: 721414
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I52c830c1d8604beee5bff591351cca3fbe0765c3
Reviewed-on: https://chromium-review.googlesource.com/940445


Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539602}