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

base/prefs/pref_service.cc

Index: base/prefs/pref_service.cc
diff --git a/base/prefs/pref_service.cc b/base/prefs/pref_service.cc
index 67ab898459459bb198cd473711f4183699143bed..b1f4f3fb16c6589e3a859314faaba63c41982a57 100644
--- a/base/prefs/pref_service.cc
+++ b/base/prefs/pref_service.cc
@@ -273,6 +273,12 @@ const base::Value* PrefService::GetUserPrefValue(const char* path) const {
   return value;
 }
 
+void PrefService::SetDefaultPrefValue(const char* path,
+                                      base::Value* value) {
+  DCHECK(CalledOnValidThread());
+  pref_registry_->SetDefaultPrefValue(path, value);
+}
+
 const base::Value* PrefService::GetDefaultPrefValue(const char* path) const {
   DCHECK(CalledOnValidThread());
   // Lookup the preference in the default store.