Add ability to change default pref values, and use in BrowserInstantController.

base/prefs/pref_service.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This provides a way to access the application's current preferences.
 
 // Chromium settings and storage represent user-selected preferences and
 // information and MUST not be extracted, overwritten or modified except
 // through Chromium defined APIs.
 
@@ skipping 202 matching lines @@
   int64 GetInt64(const char* path) const;
 
   // As above, but for unsigned values.
   void SetUint64(const char* path, uint64 value);
   uint64 GetUint64(const char* path) const;
 
   // Returns the value of the given preference, from the user pref store. If
   // the preference is not set in the user pref store, returns NULL.
   const base::Value* GetUserPrefValue(const char* path) const;
 
+  // Changes the default value for a preference. Takes ownership of |value|.
+  //
+  // Will cause a pref change notification to be fired if this causes
+  // the effective value to change.
+  void SetDefaultPrefValue(const char* path, base::Value* value);
+
   // Returns the default value of the given preference. |path| must point to a
   // registered preference. In that case, will never return NULL.
   const base::Value* GetDefaultPrefValue(const char* path) const;
 
   // Returns true if a value has been set for the specified path.
   // NOTE: this is NOT the same as FindPreference. In particular
   // FindPreference returns whether RegisterXXX has been invoked, where as
   // this checks if a value exists for the path.
   bool HasPrefPath(const char* path) const;
 
@@ skipping 116 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(PrefService);
 };
 
 // Retrieves a PrefService for the given context.
 //
 // TODO(joi): This doesn't really belong here, since it references a
 // content type; probably best to get rid of it completely.
 PrefService* PrefServiceFromBrowserContext(content::BrowserContext* context);
 
 #endif  // BASE_PREFS_PREF_SERVICE_H_