[Closure Externs] Add more APIs to the presubmit warning list

extensions/common/api/externs_checker.py

 # Copyright 2016 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.
 
 class ExternsChecker(object):
   _UPDATE_MESSAGE = """To update the externs, run:
  src/ $ python tools/json_schema_compiler/compiler.py\
  %s --root=. --generator=externs > %s"""
 
   def __init__(self, input_api, output_api, api_pairs):
     self._input_api = input_api
     self._output_api = output_api
     self._api_pairs = api_pairs
 
+    for path in api_pairs.keys() + api_pairs.values():
+      if not input_api.os_path.exists(path):
+        raise OSError('Path Not Found: %s' % path)
+
   def RunChecks(self):
     bad_files = []
     affected = [f.AbsoluteLocalPath() for f in self._input_api.AffectedFiles()]
     for path in affected:
       pair = self._api_pairs.get(path)
       if pair != None and pair not in affected:
         bad_files.append({'source': path, 'extern': pair})
     results = []
     if bad_files:
       replacements = (('<source_file>', '<output_file>') if len(bad_files) > 1
           else (bad_files[0]['source'], bad_files[0]['extern']))
       long_text = self._UPDATE_MESSAGE % replacements
       results.append(self._output_api.PresubmitPromptWarning(
           str('Found updated extension api files without updated extern files. '
               'Please update the extern files.'),
           [f['source'] for f in bad_files],
           long_text))
     return results