Fix problem where field trials using kExpirationYearInFuture could get disabled.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
===================================================================
--- base/metrics/field_trial.h	(revision 175570)
+++ base/metrics/field_trial.h	(working copy)
@@ -283,8 +283,9 @@
   // second process to mimic our state (i.e., provide the same group name).
   static const char kPersistentStringSeparator;  // Currently a slash.
 
-  // Define expiration year in future. It is initialized to two years from Now.
-  static int kExpirationYearInFuture;
+  // Year that is guaranteed to not be expired when instantiating a field trial
+  // via |FactoryGetFieldTrial()|.  Set to two years from the build date.
+  static int kNoExpirationYear;
 
   // Observer is notified when a FieldTrial's group is selected.
   class BASE_EXPORT Observer {