Log the freshness of the Variations seed in a histogram.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index b99aaed05ffbe1862b95781f1c8ab672a31200eb..599418ecae17d3f2eb348e412471896ba50a008e 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -1276,6 +1276,10 @@ const char kBookmarkPromptEnabled[] = "bookmark_prompt_enabled";
 const char kBookmarkPromptImpressionCount[] =
     "bookmark_prompt_impression_count";
 
+// 64-bit integer serialization of the base::Time from the last successful seed
+// fetch (i.e. when the Variations server responds with 200 or 304).
+const char kVariationsLastFetchTime[] = "variations_last_fetch_time";
+
 // String serialized form of variations seed protobuf.
 const char kVariationsSeed[] = "variations_seed";