Introduce a temporary workaround to ensure tracked preferences set to empty list/dict are not confu…

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 a4a4f33294c741b278a76ac116e2e02da40d3f0f..6d2476292a9c0bf037be468e18100b81e3f0c686 100644
--- a/chrome/browser/prefs/pref_metrics_service.cc
+++ b/chrome/browser/prefs/pref_metrics_service.cc
@@ -92,6 +92,8 @@ PrefMetricsService::PrefMetricsService(Profile* profile)
 
   RegisterSyncedPrefObservers();
 
+  MarkNeedsEmptyValueForTrackedPreferences();
+
   // The following code might cause callbacks into this instance before we exit
   // the constructor. This instance should be initialized at this point.
 #if defined(OS_WIN) || defined(OS_MACOSX)
@@ -120,6 +122,7 @@ PrefMetricsService::PrefMetricsService(Profile* profile,
       tracked_pref_path_count_(tracked_pref_path_count),
       checked_tracked_prefs_(false),
       weak_factory_(this) {
+  MarkNeedsEmptyValueForTrackedPreferences();
   CheckTrackedPreferences();
 }
 
@@ -266,6 +269,19 @@ void PrefMetricsService::GetDeviceIdCallback(const std::string& device_id) {
     CheckTrackedPreferences();
 }
 
+void PrefMetricsService::MarkNeedsEmptyValueForTrackedPreferences() {
+  for (int i = 0; i < tracked_pref_path_count_; ++i) {
+    // Skip prefs that haven't been registered.
+    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]);
+  }
+}
+
 // To detect changes to Preferences that happen outside of Chrome, we hash
 // selected pref values and save them in local state. CheckTrackedPreferences
 // compares the saved values to the values observed in the profile's prefs. A