Modernize some extensions code.

extensions/browser/extension_event_histogram_value.h

 // Copyright 2015 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.
 
 #ifndef EXTENSIONS_BROWSER_EXTENSION_EVENT_HISTOGRAM_VALUE_H_
 #define EXTENSIONS_BROWSER_EXTENSION_EVENT_HISTOGRAM_VALUE_H_
 
 namespace extensions {
 namespace events {
 
-// TODO(kalman): I am still in the process of migrating Event construction away
-// from using "UNKNOWN" to their real histogram values. See crbug.com/503402.
-//
 // Short version:
 //  *Never* reorder or delete entries in the |HistogramValue| enumeration.
 //  When creating a new extension event, add a new entry at the end of the
 //  enum, just prior to ENUM_BOUNDARY.
 //
 // Long version: See extension_function_histogram_value.h
 enum HistogramValue {
   UNKNOWN = 0,
   FOR_TEST,  // Tests should use this for a stub histogram value (not UNKNOWN).
   ACCESSIBILITY_PRIVATE_ON_INTRODUCE_CHROME_VOX,
@@ skipping 404 matching lines @@
   ACCESSIBILITY_PRIVATE_ON_TWO_FINGER_TOUCH_STOP,
   // Last entry: Add new entries above, then run:
   // python tools/metrics/histograms/update_extension_histograms.py
   ENUM_BOUNDARY
 };
 
 }  // namespace events
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_EVENT_HISTOGRAM_VALUE_H_