Move preference MACs to the protected preference stores.

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 c24af957b9fa70d84b5b73347a530e8be51f82a7..908d867dbf5e6f46e8896931b116f55d38b75048 100644
--- a/base/prefs/writeable_pref_store.h
+++ b/base/prefs/writeable_pref_store.h
@@ -26,6 +26,17 @@ class BASE_PREFS_EXPORT WriteablePrefStore : public PrefStore {
   // Removes the value for |key|.
   virtual void RemoveValue(const std::string& key) = 0;
 
+  // Equivalent to PrefStore::GetValue but returns a mutable value.
+  virtual bool GetMutableValue(const std::string& key,
+                               base::Value** result) = 0;
+
+  // Triggers a value changed notification. This function needs to be called
+  // if one retrieves a list or dictionary with GetMutableValue and change its
+  // value. SetValue takes care of notifications itself. Note that
+  // ReportValueChanged will trigger notifications even if nothing has changed.
+  virtual void ReportValueChanged(const std::string& key) = 0;
+
+
  protected:
   virtual ~WriteablePrefStore() {}