PRESUBMIT: UMA histogram check, rewind histograms.xml file between unmatched_histograms searches

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 388 matching lines @@
       histogram_name_found = _FindHistogramNameInLine(histogram_info[0], line)
       if histogram_name_found:
         break
     if not histogram_name_found:
       unmatched_histograms.append(histogram_info)
 
   problems = []
   if unmatched_histograms:
     with open('tools/metrics/histograms/histograms.xml') as histograms_xml:
       for histogram_name, f, line_num in unmatched_histograms:
+        histograms_xml.seek(0)
         histogram_name_found = False
         for line in histograms_xml:
           histogram_name_found = _FindHistogramNameInLine(histogram_name, line)
           if histogram_name_found:
             break
         if not histogram_name_found:
           problems.append(' [%s:%d] %s' %
                           (f.LocalPath(), line_num, histogram_name))
 
   if not problems:
@@ skipping 1339 matching lines @@
     # Explicitly iterate over copies of dicts since we mutate them.
     for master in builders.keys():
       for builder in builders[master].keys():
         # Do not trigger presubmit builders, since they're likely to fail
         # (e.g. OWNERS checks before finished code review), and we're
         # running local presubmit anyway.
         if 'presubmit' in builder:
           builders[master].pop(builder)
 
   return builders