Update mb.py to support gn directories with no args.gn

tools/mb/mb.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 """MB - the Meta-Build wrapper around GYP and GN
 
 MB is a wrapper script for GYP and GN that can be used to generate build files
 for sets of canned configurations and analyze them.
 """
@@ skipping 508 matching lines @@
       return vals
 
     # TODO: We can only get the config for GN build dirs, not GYP build dirs.
     # GN stores the args that were used in args.gn in the build dir,
     # but GYP doesn't store them anywhere. We should consider modifying
     # gyp_chromium to record the arguments it runs with in a similar
     # manner.
 
     mb_type_path = self.PathJoin(self.ToAbsPath(build_dir), 'mb_type')
     if not self.Exists(mb_type_path):
-      gn_args_path = self.PathJoin(self.ToAbsPath(build_dir), 'args.gn')
-      if not self.Exists(gn_args_path):
+      toolchain_path = self.PathJoin(self.ToAbsPath(build_dir),
+                                     'toolchain.ninja')
+      if not self.Exists(toolchain_path):
         self.Print('Must either specify a path to an existing GN build dir '
                    'or pass in a -m/-b pair or a -c flag to specify the '
                    'configuration')
         return {}
       else:
         mb_type = 'gn'
     else:
       mb_type = self.ReadFile(mb_type_path).strip()
 
     if mb_type == 'gn':
       vals = self.GNValsFromDir(build_dir)
     else:
       vals = {}
     vals['type'] = mb_type
 
     return vals
 
   def GNValsFromDir(self, build_dir):
-    args_contents = self.ReadFile(
-        self.PathJoin(self.ToAbsPath(build_dir), 'args.gn'))
+    args_contents = ""
+    gn_args_path = self.PathJoin(self.ToAbsPath(build_dir), 'args.gn')
+    if self.Exists(gn_args_path):
+      args_contents = self.ReadFile(gn_args_path)
     gn_args = []
     for l in args_contents.splitlines():
       fields = l.split(' ')
       name = fields[0]
       val = ' '.join(fields[2:])
       gn_args.append('%s=%s' % (name, val))
 
     return {
       'gn_args': ' '.join(gn_args),
       'type': 'gn',
@@ skipping 905 matching lines @@
   # Then check to see if the arg contains any metacharacters other than
   # double quotes; if it does, quote everything (including the double
   # quotes) for safety.
   if any(a in UNSAFE_FOR_CMD for a in arg):
     arg = ''.join('^' + a if a in ALL_META_CHARS else a for a in arg)
   return arg
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))