Remove JsonPrefStore pruning of empty values on write.

chrome/browser/prefs/pref_metrics_service.cc

Index: chrome/browser/prefs/pref_metrics_service.cc
diff --git a/chrome/browser/prefs/pref_metrics_service.cc b/chrome/browser/prefs/pref_metrics_service.cc
index 0bc8c1779dde865207af00007088761fd33eecf0..3fbdaf4e6817f2e686c3ec395323988fa92b6fe0 100644
--- a/chrome/browser/prefs/pref_metrics_service.cc
+++ b/chrome/browser/prefs/pref_metrics_service.cc
@@ -299,11 +299,6 @@ void PrefMetricsService::CheckTrackedPreferences() {
     if (!prefs_->FindPreference(tracked_pref_paths_[i]))
       continue;
 
-    // Make sure tracked prefs are saved to disk even if empty.
-    // TODO(gab): Guarantee this for all prefs at a lower level and remove this
-    // hack.
-    prefs_->MarkUserStoreNeedsEmptyValue(tracked_pref_paths_[i]);
-
     const base::Value* value = prefs_->GetUserPrefValue(tracked_pref_paths_[i]);
     std::string last_hash;
     // First try to get the stored expected hash...