Add UMA tracking for sync.max_invalidation_versions pref migration.

chrome/browser/sync/invalidations/invalidator_storage.cc

Index: chrome/browser/sync/invalidations/invalidator_storage.cc
diff --git a/chrome/browser/sync/invalidations/invalidator_storage.cc b/chrome/browser/sync/invalidations/invalidator_storage.cc
index defe2570b26189c5231f70f992ba2472aa82f01f..4ca9f2dc68109345609afc774cc44ce968cce015 100644
--- a/chrome/browser/sync/invalidations/invalidator_storage.cc
+++ b/chrome/browser/sync/invalidations/invalidator_storage.cc
@@ -6,6 +6,7 @@
 
 #include "base/base64.h"
 #include "base/logging.h"
+#include "base/metrics/histogram.h"
 #include "base/string_number_conversions.h"
 #include "base/values.h"
 #include "chrome/browser/prefs/pref_service.h"
@@ -160,6 +161,11 @@ void InvalidatorStorage::MigrateMaxInvalidationVersionsPref() {
     SerializeToList(max_versions, &max_versions_list);
     pref_service_->Set(prefs::kInvalidatorMaxInvalidationVersions,
                        max_versions_list);
+    UMA_HISTOGRAM_BOOLEAN("InvalidatorStorage.MigrateInvalidationVersionsPref",
+                          true);
+  } else {
+    UMA_HISTOGRAM_BOOLEAN("InvalidatorStorage.MigrateInvalidationVersionsPref",
+                          false);
   }
   pref_service_->ClearPref(prefs::kSyncMaxInvalidationVersions);
 }