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..ea4e9ddc6514c8d0074beacec5a2f66e75ac126a 100644
--- a/tools/metrics/histograms/update_histogram_enum.py
+++ b/tools/metrics/histograms/update_histogram_enum.py
@@ -142,6 +142,29 @@ def UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
     enum_node.appendChild(new_item_nodes[value])
 
 
+def HistogramNeedsUpdate(histogram_enum_name, source_enum_path, start_marker,
+                         end_marker):
+  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()
+
+  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()
+
+  Log('Comparing histograms enum with new enum definition.')
+  UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
+                             source_enum_path, histograms_doc)
+
+  new_xml = print_style.GetPrintStyle().PrettyPrintNode(histograms_doc)

[Ilya Sherman, 2016/03/02 00:59:29]

It looks like you copy/pasted lines 151-163 here.  Please factor out this shared
code into a helper function.

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

Done. It's a bit messy, but I haven't been able to think of a better way of
organizing it, given that UpdateHistogramEnum, ReadHistogramValues and
UpdateHistogramFromDict are all used by other code.

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

Thanks!  This is the structure that I was thinking of as well -- I agree that
it's a bit awkward, but IMO it's still nicer than duplicating the code =)

+
+  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