Add comment to metrics.h

webrtc/system_wrappers/include/metrics.h

 //
 // Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 //
 // Use of this source code is governed by a BSD-style license
 // that can be found in the LICENSE file in the root of the source
 // tree. An additional intellectual property rights grant can be found
 // in the file PATENTS.  All contributing project authors may
 // be found in the AUTHORS file in the root of the source tree.
 //
 
@@ skipping 40 matching lines @@
 //
 // RTC_HISTOGRAM_COUNTS("WebRTC.Video.NacksSent", nacks_sent, 1, 100000, 100);
 //
 // enum Types {
 //   kTypeX,
 //   kTypeY,
 //   kBoundary,
 // };
 //
 // RTC_HISTOGRAM_ENUMERATION("WebRTC.Types", kTypeX, kBoundary);
+//
+// NOTE: It is recommended to do the Chromium review for modifications to
+// histograms.xml before new metrics are committed to WebRTC.
 
 
 // Macros for adding samples to a named histogram.
 
 // Histogram for counters (exponentially spaced buckets).
 #define RTC_HISTOGRAM_COUNTS_100(name, sample) \
   RTC_HISTOGRAM_COUNTS(name, sample, 1, 100, 50)
 
 #define RTC_HISTOGRAM_COUNTS_200(name, sample) \
   RTC_HISTOGRAM_COUNTS(name, sample, 1, 200, 50)
@@ skipping 162 matching lines @@
 const std::string& GetHistogramName(Histogram* histogram_pointer);
 
 // Function for adding a |sample| to a histogram.
 void HistogramAdd(Histogram* histogram_pointer, int sample);
 
 }  // namespace metrics
 }  // namespace webrtc
 
 #endif  // WEBRTC_SYSTEM_WRAPPERS_INCLUDE_METRICS_H_