BUILD.gn: make the translator fuzzer a source set 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/386636 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>