Fix race and error case in GN auto-roller script.

tools/gn/bin/roll_gn.py

 # Copyright 2014 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.
 
 """An auto-roller for GN binaries into Chromium.
 
 This script is used to update the GN binaries that a Chromium
 checkout uses. In order to update the binaries, one must follow
 four steps in order:
 
@@ skipping 163 matching lines @@
       deps_content = fp.read()
       new_deps = deps_content.replace("'buildtools_revision':",
                                       "'buildtools_revision':  ")
 
     with open('DEPS', 'w') as fp:
       fp.write(new_deps)
 
   def WaitForBuildToFinish(self):
     print('Checking build')
     results = self.CheckBuild()
-    while any(r['state'] == 'pending' for r in results.values()):
+    while (len(results) or
+           any(r['state'] == 'pending' for r in results.values())):
       print()
       print('Sleeping for 30 seconds')
       time.sleep(30)
       print('Checking build')
       results = self.CheckBuild()
     return 0 if all(r['state'] == 'success' for r in results.values()) else 1
 
   def CheckBuild(self):
     _, out, _ = self.Call('git-cl issue')
 
@@ skipping 11 matching lines @@
 
     try:
       patchset_data = json.loads(rpc_server.Send('/api/%d/%d' %
                                                  (issue, patchset)))
     except Exception as _e:
       raise
 
     try_job_results = patchset_data['try_job_results']
     if not try_job_results:
       print('No try jobs found on most recent patchset')
-      return 1
+      return {}
 
     results = {}
     for job in try_job_results:
       builder = job['builder']
       if builder == 'linux_chromium_gn_upload':
         platform = 'linux64'
       elif builder == 'mac_chromium_gn_upload':
         platform = 'mac'
       elif builder == 'win8_chromium_gn_upload':
         platform = 'win'
@@ skipping 178 matching lines @@
   def Call(self, cmd, cwd=None):
     proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell=True,
                             cwd=(cwd or self.chromium_src_dir))
     out, err = proc.communicate()
     return proc.returncode, out, err
 
 
 if __name__ == '__main__':
   roller = GNRoller()
   sys.exit(roller.Roll())