Make the SYSLOG presubmit check only check changed lines.

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 depot_tools.
 """
 
@@ skipping 2260 matching lines @@
         '\n'.join(problems))]
 
   return []
 
 
 def _CheckSyslogUseWarning(input_api, output_api, source_file_filter=None,
                            lint_filters=None, verbose_level=None):
   """Checks that all source files use SYSLOG properly."""
   syslog_files = []
   for f in input_api.AffectedSourceFiles(source_file_filter):
-    if 'SYSLOG' in input_api.ReadFile(f, 'rb'):
-      syslog_files.append(f.LocalPath())
+    for line_number, line in f.ChangedContents():
+      if 'SYSLOG' in line:
+        syslog_files.append(f.LocalPath() + ':' + str(line_number))
+
   if syslog_files:
     return [output_api.PresubmitPromptWarning(
         'Please make sure there are no privacy sensitive bits of data in SYSLOG'
         ' calls.\nFiles to check:\n', items=syslog_files)]
   return []
 
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
@@ skipping 50 matching lines @@
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results