Touch up md_history presubmit

chrome/browser/resources/md_history/PRESUBMIT.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.
 
-import os.path
-import time
 
 def CheckChangeOnUpload(input_api, output_api):
   """Warn when changing md_history without vulcanizing."""
 
-  def _is_md_history_file(path):
-    return (path.startswith('chrome/browser/resources/md_history') and
-            (not path.endswith('externs.js')) and
-            (not path.endswith('crisper.js')) and
-            (not path.endswith('vulcanized.html')) and
-            (path.endswith('js') or path.endswith('html')))
+  def _is_history_source_file(file):
+    path = file.LocalPath()
+    return (not path.endswith('externs.js') and
+            not path.endswith('crisper.js') and
+            not path.endswith('vulcanized.html') and
+            (path.endswith('.js') or path.endswith('.html')))
 
-  paths = [x.LocalPath() for x in input_api.change.AffectedFiles()]
-  earliest_vulcanize_change = min(os.path.getmtime(x) for x in
+  os_path = input_api.os_path
+  earliest_vulcanize_change = min(os_path.getmtime(x) for x in
                                   ['app.vulcanized.html',
                                    'app.crisper.js',
                                    'lazy_load.vulcanized.html',
                                    'lazy_load.crisper.js'])
-  history_changes = filter(_is_md_history_file, paths)
+
+  source_files = input_api.AffectedFiles(file_filter=_is_history_source_file)
   latest_history_change = 0
-  if history_changes:
-    latest_history_change = max(os.path.getmtime(os.path.split(x)[1]) for x in
-                                history_changes)
+  if source_files:
+    latest_history_change = max(
+        os_path.getmtime(os_path.basename(f.LocalPath())) for f in source_files)
 
   if latest_history_change > earliest_vulcanize_change:
     return [output_api.PresubmitPromptWarning(
         'Vulcanize must be run when changing files in md_history. See '
         'docs/vulcanize.md.')]
   return []