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

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index c6d0396bbd4397af6021f0eee0c1d2f8afe19aa7..cd0e8e494d7325a35bb74f59222c5c83a3e40345 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -625,14 +625,6 @@ group("gn_only") {
     ]
   }
 
-  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
@@ -652,8 +644,6 @@ group("gn_only") {
       "//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",
@@ -721,6 +711,14 @@ group("gn_only") {
     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" ]
   }