Add tcmalloc to GN build

tools/gn.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project 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 subprocess
 import sys
 import os
 import utils
@@ skipping 51 matching lines @@
   # checked-in sdk, this can be removed.
   gn_args['dart_host_pub_exe'] = os.path.join(
       DART_ROOT, 'tools', 'sdks', HOST_OS, 'dart-sdk', 'bin', 'pub')
 
   # For Fuchsia support, the default is to not compile in the root
   # certificates.
   gn_args['dart_use_fallback_root_certificates'] = True
 
   gn_args['dart_zlib_path'] = "//runtime/bin/zlib"
 
+  gn_args['dart_use_tcmalloc'] = gn_args['target_os'] == 'linux'
+
   gn_args['is_debug'] = args.mode == 'debug'
   gn_args['is_release'] = args.mode == 'release'
   gn_args['is_product'] = args.mode == 'product'
   gn_args['dart_debug'] = args.mode == 'debug'
 
   # This setting is only meaningful for Flutter. Standalone builds of the VM
   # should leave this set to 'develop', which causes the build to defer to
   # 'is_debug', 'is_release' and 'is_product'.
   gn_args['dart_runtime_mode'] = 'develop'
 
@@ skipping 68 matching lines @@
   ]
   gn_args = to_command_line(to_gn_args(args))
   out_dir = get_out_dir(args)
   print "gn gen --check in %s" % out_dir
   command.append(out_dir)
   command.append('--args=%s' % ' '.join(gn_args))
   return subprocess.call(command, cwd=DART_ROOT)
 
 if __name__ == '__main__':
     sys.exit(main(sys.argv))