Mark as unused the "Closing" UMA entry in ImportantFile.TempFileFailures.

BUG=none

Review URL: https://codereview.chromium.org/689673002

Cr-Commit-Position: refs/heads/master@{#308341}

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,

tools/metrics/histograms/histograms.xml

@@ -56928,7 +56928,7 @@ To add a new entry, add it with any value and run test to compute valid value.
 <enum name="TempFileFailure" type="int">
   <int value="0" label="Creating"/>
   <int value="1" label="Opening"/>
-  <int value="2" label="Closing"/>
+  <int value="2" label="Closing (unused)"/>
   <int value="3" label="Writing"/>
   <int value="4" label="Renaming"/>
   <int value="5" label="Flushing"/>