Update from https://crrev.com/308996

base/files/important_file_writer.cc

 // Copyright (c) 2011 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.
 
 #include "base/files/important_file_writer.h"
 
 #include <stdio.h>
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/critical_closure.h"
 #include "base/files/file.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/task_runner.h"
 #include "base/task_runner_util.h"
 #include "base/threading/thread.h"
 #include "base/time/time.h"
 
 namespace base {
 
 namespace {
 
 const int kDefaultCommitIntervalMs = 10000;
 
+// This enum is used to define the buckets for an enumerated UMA histogram.
+// Hence,
+//   (a) existing enumerated constants should never be deleted or reordered, and
+//   (b) new constants should only be appended at the end of the enumeration.
 enum TempFileFailure {
   FAILED_CREATING,
   FAILED_OPENING,
-  FAILED_CLOSING,
+  FAILED_CLOSING,  // Unused.
   FAILED_WRITING,
   FAILED_RENAMING,
   FAILED_FLUSHING,
   TEMP_FILE_FAILURE_MAX
 };
 
 void LogFailure(const FilePath& path, TempFileFailure failure_code,
                 const std::string& message) {
   UMA_HISTOGRAM_ENUMERATION("ImportantFile.TempFileFailures", failure_code,
                             TEMP_FILE_FAILURE_MAX);
@@ skipping 149 matching lines @@
 
 void ImportantFileWriter::ForwardSuccessfulWrite(bool result) {
   DCHECK(CalledOnValidThread());
   if (result && !on_next_successful_write_.is_null()) {
     on_next_successful_write_.Run();
     on_next_successful_write_.Reset();
   }
 }
 
 }  // namespace base