When we want to use GN standalone, the fuzzer_test template won't exist.
Instead make the fuzzer a source set that is used by a target in Chrome.

BUG=angleproject:1522

Change-Id: I347e633448d508eb124d0861b0fb28315923016c
Reviewed-on: https://chromium-review.googlesource.com/386636Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>