[libfuzzer] replace static_library with source_set

testing/libfuzzer/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # LibFuzzer is a LLVM tool for coverage-guided fuzz testing.
 # See http://www.chromium.org/developers/testing/libfuzzer
 #
 # To enable libfuzzer, 'use_libfuzzer' GN option should be set to true.
 
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
-static_library("libfuzzer_main") {
+source_set("libfuzzer_main") {
   deps = []
   sources = []
   if (use_libfuzzer) {
     deps += [ "//third_party/libFuzzer:libfuzzer" ]
   } else if (use_drfuzz) {
     sources += [ "drfuzz_main.cc" ]
   } else {
     sources += [ "unittest_main.cc" ]
   }
 }
 
 # Noop config used to tag fuzzer tests excluded from clusterfuzz.
 # Libfuzzer build bot uses this to filter out targets while
 # building an archive for clusterfuzz.
 config("no_clusterfuzz") {
 }
 
 # noop to tag seed corpus rules.
 source_set("seed_corpus") {
 }