Keep emtpy List/Dictionary pref value with non-empty default.

chrome/browser/prefs/pref_service_mock_builder.h

Index: chrome/browser/prefs/pref_service_mock_builder.h
diff --git a/chrome/browser/prefs/pref_service_mock_builder.h b/chrome/browser/prefs/pref_service_mock_builder.h
index 5ea5ace2832a247dd220e4a471b298b09cb360cc..2dd764f1786886b030f035f7da47d3a4e8cd7550 100644
--- a/chrome/browser/prefs/pref_service_mock_builder.h
+++ b/chrome/browser/prefs/pref_service_mock_builder.h
@@ -15,6 +15,10 @@ class CommandLine;
 class FilePath;
 class PrefService;
 
+namespace base {
+class MessageLoopProxy;
+}
+
 namespace policy {
 class PolicyService;
 }
@@ -46,6 +50,9 @@ class PrefServiceMockBuilder {
 
   // Specifies to use an actual file-backed user pref store.
   PrefServiceMockBuilder& WithUserFilePrefs(const FilePath& prefs_file);
+  PrefServiceMockBuilder& WithUserFilePrefs(
+      const FilePath& prefs_file,
+      base::MessageLoopProxy* message_loop_proxy);
 
   // Creates the PrefService, invalidating the entire builder configuration.
   PrefService* Create();