Make the SYSLOG presubmit check only check changed lines.

The check as it is now was spamming too many unrelated changes.
Also make output of the check more precise by showing the line
number affected.

BUG=none

Review-Url: https://codereview.chromium.org/2622083006
Cr-Commit-Position: refs/heads/master@{#443200}

PRESUBMIT.py

@@ -2278,8 +2278,10 @@ def _CheckSyslogUseWarning(input_api, output_api, source_file_filter=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'