[AFL][libprotobuf-mutator] Prevent build bots from building proto fuzzers for AFL

Bug: 769582
Change-Id: I197d6ac177dd14e00faaa2ab0a54b72576197c9c
Reviewed-on: https://chromium-review.googlesource.com/694641Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506543}

third_party/libprotobuf-mutator/BUILD.gn

 # Copyright 2017 The Chromium Authors. All rights reserved.
+
+import("//testing/libfuzzer/fuzzer_test.gni")
+
 config("include_config") {
   include_dirs = [ "src/" ]
 }
@@ -18,4 +21,10 @@ source_set("libprotobuf-mutator") {
   deps = [
     "//third_party/protobuf:protobuf_full",
   ]
+
+  # Let ClusterFuzz builders know to not build targets that depend on 
+  # libprotobuf-mutator for AFL.
+  if (use_afl) {
+    all_dependent_configs = [ "//testing/libfuzzer:no_clusterfuzz" ]
+  }
 }