Add histograms presubmit check for UseCounter.h

tools/metrics/histograms/update_histogram_enum.py

Index: tools/metrics/histograms/update_histogram_enum.py
diff --git a/tools/metrics/histograms/update_histogram_enum.py b/tools/metrics/histograms/update_histogram_enum.py
index ba975996719ce541a07445cf4a3a4c4cefb4cd23..d2f9617571e0608424d0ca302a3b8baf209324e1 100644
--- a/tools/metrics/histograms/update_histogram_enum.py
+++ b/tools/metrics/histograms/update_histogram_enum.py
@@ -142,17 +142,10 @@ def UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
     enum_node.appendChild(new_item_nodes[value])
 
 
-def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
-                            source_enum_path):
-  """Updates |histogram_enum_name| enum in histograms.xml file with values
-  from the {value: 'key'} dictionary |source_enum_values|. A comment is added
-  to histograms.xml citing that the values in |histogram_enum_name| were
-  sourced from |source_enum_path|.
-  """
-  HISTOGRAMS_PATH = path_util.GetHistogramsFile()
-
-  Log('Reading existing histograms from "{0}".'.format(HISTOGRAMS_PATH))
-  with open(HISTOGRAMS_PATH, 'rb') as f:
+def _GetOldAndUpdatedXml(histograms_path, histogram_enum_name,
+                         source_enum_values, source_enum_path):

[Ilya Sherman, 2016/03/02 23:18:46]

Please add documentation for this function.

[suzyh_UTC10 (ex-contributor), 2016/03/02 23:49:16]

Done.

+  Log('Reading existing histograms from "{0}".'.format(histograms_path))
+  with open(histograms_path, 'rb') as f:
     histograms_doc = minidom.parse(f)
     f.seek(0)
     xml = f.read()
@@ -161,8 +154,45 @@ def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
   UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
                              source_enum_path, histograms_doc)
 
-  Log('Writing out new histograms file.')
   new_xml = print_style.GetPrintStyle().PrettyPrintNode(histograms_doc)
+

[Ilya Sherman, 2016/03/02 23:18:46]

Optional nit: I'd omit this newline.

[suzyh_UTC10 (ex-contributor), 2016/03/02 23:49:17]

Done.

+  return (xml, new_xml)
+
+
+def HistogramNeedsUpdate(histogram_enum_name, source_enum_path, start_marker,
+                         end_marker):
+  """Reads a C++ enum from a .h file and does a dry run of updating
+  histograms.xml to match. Returns true if the histograms.xml file would be
+  changed.
+
+  Args:
+      histogram_enum_name: The name of the XML <enum> attribute to update.
+      source_enum_path: A unix-style path, relative to src/, giving
+          the C++ header file from which to read the enum.
+      start_marker: A regular expression that matches the start of the C++ enum.
+      end_marker: A regular expression that matches the end of the C++ enum.
+  """
+  Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
+  source_enum_values = ReadHistogramValues(source_enum_path, start_marker,
+                                           end_marker)
+
+  HISTOGRAMS_PATH = path_util.GetHistogramsFile()
+  (xml, new_xml) = _GetOldAndUpdatedXml(HISTOGRAMS_PATH, histogram_enum_name,
+                                        source_enum_values, source_enum_path)
+  return xml != new_xml
+
+
+def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
+                            source_enum_path):
+  """Updates |histogram_enum_name| enum in histograms.xml file with values
+  from the {value: 'key'} dictionary |source_enum_values|. A comment is added
+  to histograms.xml citing that the values in |histogram_enum_name| were
+  sourced from |source_enum_path|.
+  """
+  HISTOGRAMS_PATH = path_util.GetHistogramsFile()

[Ilya Sherman, 2016/03/02 23:18:46]

nit: I think this can be moved into _GetOldAndUpdatedXml

[suzyh_UTC10 (ex-contributor), 2016/03/02 23:27:10]

I contemplated this, but then _GetOldAndUpdatedXml would also have to return
HISTOGRAMS_PATH, because it is needed below. Which would you prefer?

[Ilya Sherman, 2016/03/02 23:35:31]

Ah, I missed that.  I think it would be fine to promote HISTOGRAMS_PATH to be a
file-level constant, so that it'd be accessible from any method.

[suzyh_UTC10 (ex-contributor), 2016/03/02 23:49:17]

Done.

+
+  (xml, new_xml) = _GetOldAndUpdatedXml(HISTOGRAMS_PATH, histogram_enum_name,
+                                        source_enum_values, source_enum_path)
   if not diff_util.PromptUserToAcceptDiff(
       xml, new_xml, 'Is the updated version acceptable?'):
     Log('Cancelled.')