Enable a NetworkService unit test for ChromeOS and Fuchsia.

This test was initially disabled for them since they didn't have a
NetworkChangeNotifier implementation.

TBR=cmumford@google.com

Change-Id: I1e79f8dd6a7c9c3c596f990a88ec6db7539f76f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532900
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642753}