[layout-test] Fix netinfo/gc-unused-listener test to use asyncGC

We should use asyncGC to ensure we do a full GC not just V8 GC.

Bug: chromium:976587
Change-Id: I3f9462b4ffd26f97827b5b48d2f3d4b8ed3dfa2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669304
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671239}
2 files changed