metrics_daemon: fix harmless confusion in writing persistent integers

As the subject says, I don't think this can change any of the
behavior, but the argument to Write() was ignored and merely
confusing.

BUG=chromium:339588
TEST=ran unit tests

Change-Id: Ie6d59e6879a734523978c21d66170721e0f499c7
Reviewed-on: https://chromium-review.googlesource.com/194919
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
diff --git a/metrics/persistent_integer.h b/metrics/persistent_integer.h
index 7d920b5..5b159c7 100644
--- a/metrics/persistent_integer.h
+++ b/metrics/persistent_integer.h
@@ -45,11 +45,12 @@
  private:
   static const int kVersion = 1001;
 
-  // Writes |value| to the backing file, creating it if necessary.
-  void Write(int64 value);
+  // Writes |value_| to the backing file, creating it if necessary.
+  void Write();
 
   // Reads the value from the backing file, stores it in |value_|, and returns
-  // true if the backing file is valid.  Returns false otherwise.
+  // true if the backing file is valid.  Returns false otherwise, and creates
+  // a valid backing file as a side effect.
   bool Read();
 
   int64 value_;