Update from https://crrev.com/308996

base/files/important_file_writer.cc

Index: base/files/important_file_writer.cc
diff --git a/base/files/important_file_writer.cc b/base/files/important_file_writer.cc
index d7579abb112432f8e97be4a81d5c6b1ea1c484ba..d2562364dfe9cc4ed265b2c72a455a125661c606 100644
--- a/base/files/important_file_writer.cc
+++ b/base/files/important_file_writer.cc
@@ -27,10 +27,14 @@ 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,