Add presubmit scripts to check histograms.xml if bad_message.h changes.

tools/metrics/histograms/presubmit_bad_message_reasons.py

+# Copyright 2017 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.
+
+"""Check to see if the various BadMessage enums in histograms.xml need to be
+updated. This can be called from a chromium PRESUBMIT.py to ensure updates to
+bad_message.h also include the generated changes to histograms.xml
+"""
+
+import update_histogram_enum
+
+def PrecheckBadMessage(input_api, output_api, histogram_name):
+  source_path = ''
+
+  for f in input_api.AffectedFiles():
+    if f.LocalPath().endswith('bad_message.h'):
+      source_path = f.LocalPath()
+      break
+
+  # If bad_message.h wasn't changed, then there is nothing to check.

[Ilya Sherman, 2017/02/01 00:35:13]

nit: It would be great to clarify in this documentation why this check makes
sense, i.e. that this function will be called once per bad_message.h-containing
directory.

[dougt, 2017/02/01 02:49:06]

Done.

+  if source_path == '':
+    return []
+
+  START_MARKER='^enum (class )?BadMessageReason {'
+  END_MARKER='^BAD_MESSAGE_MAX'
+  if update_histogram_enum.HistogramNeedsUpdate(
+      histogram_enum_name=histogram_name,
+      source_enum_path=source_path,
+      start_marker=START_MARKER,
+      end_marker=END_MARKER):
+    return [output_api.PresubmitPromptWarning(
+      'bad_messages.h has been updated but histogram.xml does not '

[brettw, 2017/02/01 00:28:22]

I'm not an expert at Python style (so if you know I'm wrong please ignore me),
but I generally expect lines like this wrapped to 4 spaces. Can you double-check
what style is correct here (and in the other added Python code).

[dougt, 2017/02/01 02:49:06]

Done. The style is to indent 4 spaces (or align where approriate):

http://google.github.io/styleguide/pyguide.html?showone=Indentation#Indentation

+      'appear to be updated.\nPlease run:\n'
+      '  python tools/metrics/histograms/update_bad_message_reasons.py\n')]
+  return []