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

base/prefs/pref_registry.h

Index: base/prefs/pref_registry.h
diff --git a/base/prefs/pref_registry.h b/base/prefs/pref_registry.h
index 8f39341ce55b2fe9b43f78160e471c5d2a4b9e18..6c7eac9f59588cee215a29c2aa19cf20acd7c6fb 100644
--- a/base/prefs/pref_registry.h
+++ b/base/prefs/pref_registry.h
@@ -40,6 +40,11 @@ class BASE_PREFS_EXPORT PrefRegistry : public base::RefCounted<PrefRegistry> {
   const_iterator begin() const;
   const_iterator end() const;
 
+  // Changes the default value for a preference. Takes ownership of |value|.
+  //
+  // |pref_name| must be a previously registered preference.
+  void SetDefaultPrefValue(const char* pref_name, base::Value* value);
+
   // Exactly one callback can be set for registration. The callback
   // will be invoked each time registration has been performed on this
   // object.