Fix chromium update in chromium roll script.

tools/push-to-trunk/chromium_roll.py

 #!/usr/bin/env python
 # Copyright 2014 the V8 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 os
 import sys
 
 from common_includes import *
@@ skipping 55 matching lines @@
 
 
 class UpdateChromiumCheckout(Step):
   MESSAGE = "Update the checkout and create a new branch."
   REQUIRES = "chrome_path"
 
   def RunStep(self):
     os.chdir(self["chrome_path"])
     self.GitCheckout("master")
     self._side_effect_handler.Command("gclient", "sync --nohooks")
+    self.GitPull()
     try:
       # TODO(machenbach): Add cwd to git calls.
       os.chdir(os.path.join(self["chrome_path"], "v8"))
       self.GitFetchOrigin()
     finally:
       os.chdir(self["chrome_path"])
     self.GitCreateBranch("v8-roll-%s" % self["trunk_revision"])
 
 
 class UploadCL(Step):
@@ skipping 89 matching lines @@
       SwitchChromium,
       UpdateChromiumCheckout,
       UploadCL,
       SwitchV8,
       CleanUp,
     ]
 
 
 if __name__ == "__main__":  # pragma: no cover
   sys.exit(ChromiumRoll(CONFIG).Run())