Add simple PRESUBMIT check to ensure that all files ending with .json can

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 1044 matching lines @@
           action = 'name="{0}"'.format(action_name)
           if action not in current_actions:
             return [output_api.PresubmitPromptWarning(
               'File %s line %d: %s is missing in '
               'tools/metrics/actions/actions.xml. Please run '
               'tools/metrics/actions/extract_actions.py to update.'
               % (f.LocalPath(), line_num, action_name))]
   return []
 
 
+def _CheckJSONParsability(input_api, output_api):
+  results = []
+  file_filter = lambda f: f.LocalPath().endswith('.json')
+  for fpath in input_api.AffectedFiles(file_filter=file_filter):
+    with open(fpath.LocalPath(), 'r') as f:
+      try:
+        input_api.json.load(f)
+      except ValueError:
+        results.append(output_api.PresubmitError(
+          "File %r does not parse as valid JSON" % (fpath.LocalPath())
+        ))
+  return results
+
+
 def _CheckJavaStyle(input_api, output_api):
   """Runs checkstyle on changed java files and returns errors if any exist."""
   original_sys_path = sys.path
   try:
     sys.path = sys.path + [input_api.os_path.join(
         input_api.PresubmitLocalPath(), 'tools', 'android', 'checkstyle')]
     import checkstyle
   finally:
     # Restore sys.path to what it was before.
     sys.path = original_sys_path
@@ skipping 30 matching lines @@
   results.extend(_CheckAddedDepsHaveTargetApprovals(input_api, output_api))
   results.extend(
       input_api.canned_checks.CheckChangeHasNoTabs(
           input_api,
           output_api,
           source_file_filter=lambda x: x.LocalPath().endswith('.grd')))
   results.extend(_CheckSpamLogging(input_api, output_api))
   results.extend(_CheckForAnonymousVariables(input_api, output_api))
   results.extend(_CheckCygwinShell(input_api, output_api))
   results.extend(_CheckUserActionUpdate(input_api, output_api))
+  results.extend(_CheckJSONParsability(input_api, output_api))
 
   if any('PRESUBMIT.py' == f.LocalPath() for f in input_api.AffectedFiles()):
     results.extend(input_api.canned_checks.RunUnitTestsInDirectory(
         input_api, output_api,
         input_api.PresubmitLocalPath(),
         whitelist=[r'^PRESUBMIT_test\.py$']))
   return results
 
 
 def _CheckSubversionConfig(input_api, output_api):
@@ skipping 369 matching lines @@
     builders.extend(['cros_x86'])
 
   # The AOSP bot doesn't build the chrome/ layer, so ignore any changes to it
   # unless they're .gyp(i) files as changes to those files can break the gyp
   # step on that bot.
   if (not all(re.search('^chrome', f) for f in files) or
       any(re.search('\.gypi?$', f) for f in files)):
     builders.extend(['android_aosp'])
 
   return GetDefaultTryConfigs(builders)