Attempt to fix Mac ASAN build in GN.

build/config/sanitizers/copy_asan_runtime_mac.py

+# Copyright 2016 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.
+
+import argparse
+import os
+import shutil
+import subprocess
+import sys
+
+if __name__ == '__main__':
+  parser = argparse.ArgumentParser(
+      description='Copy the checked-in asan runtime and reset the rpath')
+  parser.add_argument('input_path', nargs=1)
+  parser.add_argument('output_path', nargs=1)
+  args = parser.parse_args()
+
+  input_file = args.input_path[0]
+  output_file = args.output_path[0]
+  basename = os.path.basename(input_file)
+  if os.path.exists(output_file):
+    os.unlink(output_file)
+  shutil.copy(input_file, output_file)
+  rv = subprocess.check_call([
+      'xcrun',
+      'install_name_tool',
+      '-add_rpath',
+      '@rpath/' + basename,
+      output_file])
+  if rv:
+    os.unlink(output_file)
+  sys.exit(rv)