Fix errors in MB when run for the first time on a new builder.

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.
 """
 
 from __future__ import print_function
 
 import argparse
 import ast
+import errno
 import json
 import os
 import pipes
 import pprint
 import shlex
 import shutil
 import sys
 import subprocess
 import tempfile
 
@@ skipping 306 matching lines @@
       ninja_targets_to_labels = ast.literal_eval(self.ReadFile(os.path.join(
           self.chromium_src_dir, 'testing', 'buildbot', 'ninja_to_gn.pyl')))
       gn_labels = []
       for target in swarming_targets:
         if not target in ninja_targets_to_labels:
           raise MBErr('test target "%s"  not found in %s' %
                       (target, '//testing/buildbot/ninja_to_gn.pyl'))
         gn_labels.append(ninja_targets_to_labels[target])
 
       gn_runtime_deps_path = self.ToAbsPath(path, 'runtime_deps')
+
+      # Since GN hasn't run yet, the build directory may not even exist.
+      self.MaybeMakeDirectory(self.ToAbsPath(path))
+
       self.WriteFile(gn_runtime_deps_path, '\n'.join(gn_labels) + '\n')
       cmd.append('--runtime-deps-list-file=%s' % gn_runtime_deps_path)
 
     ret, _, _ = self.Run(cmd)
 
     for target in swarming_targets:
       deps_path = self.ToAbsPath(path, target + '.runtime_deps')
       if not self.Exists(deps_path):
           raise MBErr('did not generate %s' % deps_path)
 
@@ skipping 379 matching lines @@
     return p.returncode, out, err
 
   def ExpandUser(self, path):
     # This function largely exists so it can be overridden for testing.
     return os.path.expanduser(path)
 
   def Exists(self, path):
     # This function largely exists so it can be overridden for testing.
     return os.path.exists(path)
 
+  def MaybeMakeDirectory(self, path):
+    try:
+      os.makedirs(path)
+    except OSError, e:
+      if e.errno != errno.EEXIST:
+        raise
+
   def ReadFile(self, path):
     # This function largely exists so it can be overriden for testing.
     with open(path) as fp:
       return fp.read()
 
   def RemoveFile(self, path):
     # This function largely exists so it can be overriden for testing.
     os.remove(path)
 
   def TempFile(self, mode='w'):
@@ skipping 14 matching lines @@
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except MBErr as e:
     print(e)
     sys.exit(1)
   except KeyboardInterrupt:
     print("interrupted, exiting", stream=sys.stderr)
     sys.exit(130)