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

base/prefs/pref_service.h

Index: base/prefs/pref_service.h
diff --git a/base/prefs/pref_service.h b/base/prefs/pref_service.h
index d9cc8f5f1ce3fe65b792c4d0921476da3f083f48..9ba78beedb425922c9f9f8f93ec8e82480637be6 100644
--- a/base/prefs/pref_service.h
+++ b/base/prefs/pref_service.h
@@ -220,6 +220,12 @@ class BASE_PREFS_EXPORT PrefService : public base::NonThreadSafe {
   // 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;