[libfuzzer] fetch libfuzzer on mac and register it in build root.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 607 matching lines @@
   if (is_win || is_linux) {
     deps += [
       ":mojo_apptests",
       "//components/mus/demo",
       "//components/mus/ws:tests",
       "//mash:all",
       "//mojo",
     ]
   }
 
-  if (is_win && use_drfuzz) {
-    # build libfuzzer fuzzers on Windows to run with Dr. Fuzz
-    deps += [
-      "//testing/libfuzzer/fuzzers",
-      "//testing/libfuzzer/tests:libfuzzer_tests",
-    ]
-  }
-
   if (is_linux && !is_chromeos && !is_chromecast) {
     # TODO(GYP): Figure out if any of these should be in gn_all
     # and figure out how cross-platform they are
     deps += [
       ":gn_mojo_targets",
       "//chrome/browser/resources:extension_resource_demo",
       "//chrome/installer/util:strings",
       "//chrome/tools/convert_dict",
       "//components/constrained_window:unit_tests",
       "//components/filesystem:unittests",
       "//components/leveldb:leveldb_service_unittests",
       "//components/metrics:serialization",
       "//components/password_manager/content/renderer:browser_tests",
       "//components/rappor:unit_tests",
       "//components/sessions:unit_tests",
       "//media/blink:media_blink_unittests",
       "//media/cast:udp_proxy",
       "//native_client/src/trusted/debug_stub:gdb_rsp_unittest",
       "//storage/browser:dump_file_system",
-      "//testing/libfuzzer/fuzzers",
-      "//testing/libfuzzer/tests:libfuzzer_tests",
       "//third_party/angle:libANGLE",
       "//third_party/angle:libEGL",
       "//third_party/angle:libGLESv2",
       "//third_party/cld_2:cld_2_dynamic_data_tool",
       "//third_party/leveldatabase:leveldb_arena_test",
       "//third_party/leveldatabase:leveldb_bloom_test",
       "//third_party/leveldatabase:leveldb_cache_test",
       "//third_party/leveldatabase:leveldb_corruption_test",
       "//third_party/leveldatabase:leveldb_crc32c_test",
       "//third_party/leveldatabase:leveldb_db_bench",
@@ skipping 47 matching lines @@
     }
     if (is_linux && current_toolchain == host_toolchain) {
       deps += [ "//v8:d8" ]
     }
   }
 
   if (use_ozone) {
     deps += [ "//ui/ozone/demo" ]
   }
 
+  if ((is_linux && !is_chromeos && !is_chromecast) || (is_win && use_drfuzz) ||
+      (use_libfuzzer && is_mac)) {
+    deps += [
+      "//testing/libfuzzer/fuzzers",
+      "//testing/libfuzzer/tests:libfuzzer_tests",
+    ]
+  }
+
   if (enable_nacl) {
     deps += [ "//native_client_sdk/src:nacl_core_sdk" ]
   }
 
   if (is_android) {
     deps += [
       "//build/android/gyp/test:hello_world",
       "//build/android/incremental_install:bootstrap_java",
     ]
   }
@@ skipping 169 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }