Leftover cleanups from https://codereview.chromium.org/324493002 .

base/prefs/writeable_pref_store.h

Index: base/prefs/writeable_pref_store.h
diff --git a/base/prefs/writeable_pref_store.h b/base/prefs/writeable_pref_store.h
index 908d867dbf5e6f46e8896931b116f55d38b75048..d320dcf7360bb666ceb86f28276763e7d2311516 100644
--- a/base/prefs/writeable_pref_store.h
+++ b/base/prefs/writeable_pref_store.h
@@ -36,6 +36,11 @@ class BASE_PREFS_EXPORT WriteablePrefStore : public PrefStore {
   // ReportValueChanged will trigger notifications even if nothing has changed.
   virtual void ReportValueChanged(const std::string& key) = 0;
 
+  // Same as SetValue, but doesn't generate notifications. This is used by
+  // PrefService::GetMutableUserPref() in order to put empty entries
+  // into the user pref store. Using SetValue is not an option since existing
+  // tests rely on the number of notifications generated.
+  virtual void SetValueSilently(const std::string& key, base::Value* value) = 0;
 
  protected:
   virtual ~WriteablePrefStore() {}