GN: Pass a better -j option when using goma

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 import optparse
 import os
 import re
@@ skipping 398 matching lines @@
     '-a', arch,
     '--os', gn_os,
     '-v',
   ]
   process = subprocess.Popen(gn_command)
   process.wait()
   if process.returncode != 0:
     print ("Tried to run GN, but it failed. Try running it manually: \n\t$ " +
            ' '.join(gn_command))
 
+
+def ShouldRunGN(out_dir):
+  return (not os.path.exists(out_dir) or
+          not os.path.isfile(os.path.join(out_dir, 'args.gn')))
+
+
+def UseGoma(out_dir):
+  args_gn = os.path.join(out_dir, 'args.gn')
+  return 'use_goma = true' in open(args_gn, 'r').read()
+
+
 def BuildNinjaCommand(options, target, target_os, mode, arch):
   out_dir = utils.GetBuildRoot(HOST_OS, mode, arch, target_os)
-  if not os.path.exists(out_dir):
+  if ShouldRunGN(out_dir):
     RunGN(target_os, mode, arch)
   command = ['ninja', '-C', out_dir]
   if options.verbose:
     command += ['-v']
+  if UseGoma(out_dir):
+    command += ['-j200']
   command += [target]
   return command
 
 
 filter_xcodebuild_output = False
 def BuildOneConfig(options, target, target_os, mode, arch, override_tools):
   global filter_xcodebuild_output
   start_time = time.time()
   args = []
   build_config = utils.GetBuildConf(mode, arch, target_os)
@@ skipping 160 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())