Remove JsonPrefStore pruning of empty values on write.

chrome/browser/prefs/pref_metrics_service.h

Index: chrome/browser/prefs/pref_metrics_service.h
diff --git a/chrome/browser/prefs/pref_metrics_service.h b/chrome/browser/prefs/pref_metrics_service.h
index c776b07f2a8d56b74f9fb5907d3df77b209fdff3..d38a006d660237ff0d9625d2cffd06cacbae2170 100644
--- a/chrome/browser/prefs/pref_metrics_service.h
+++ b/chrome/browser/prefs/pref_metrics_service.h
@@ -95,10 +95,6 @@ class PrefMetricsService : public BrowserContextKeyedService {
   // Callback to receive a unique device_id.
   void GetDeviceIdCallback(const std::string& device_id);
 
-  // Marks all tracked preferences as required to save their values even if
-  // empty.
-  void MarkNeedsEmptyValueForTrackedPreferences();
-
   // Checks the tracked preferences against their last known values and reports
   // any discrepancies. This must be called after |device_id| has been set.
   void CheckTrackedPreferences();