Introduce per-tab preferences service.

chrome/browser/prefs/pref_model_associator.cc

Index: chrome/browser/prefs/pref_model_associator.cc
diff --git a/chrome/browser/prefs/pref_model_associator.cc b/chrome/browser/prefs/pref_model_associator.cc
index 40f2dff3f32a2bc8e5253c9b6ced5c70682b51df..df85cbe676673b23e5e444288b663a8b59c932bc 100644
--- a/chrome/browser/prefs/pref_model_associator.cc
+++ b/chrome/browser/prefs/pref_model_associator.cc
@@ -23,14 +23,6 @@ PrefModelAssociator::PrefModelAssociator()
       processing_syncer_changes_(false),
       pref_service_(NULL),
       sync_processor_(NULL) {
-}
-
-PrefModelAssociator::PrefModelAssociator(
-    PrefService* pref_service)
-    : models_associated_(false),
-      processing_syncer_changes_(false),
-      pref_service_(pref_service),
-      sync_processor_(NULL) {
   DCHECK(CalledOnValidThread());
 }
 
@@ -119,6 +111,7 @@ SyncError PrefModelAssociator::MergeDataAndStartSyncing(
     SyncChangeProcessor* sync_processor) {
   DCHECK_EQ(type, PREFERENCES);
   DCHECK(CalledOnValidThread());
+  DCHECK(pref_service_);
   DCHECK(!sync_processor_);
   sync_processor_ = sync_processor;
 
@@ -430,3 +423,8 @@ void PrefModelAssociator::ProcessPrefChange(const std::string& name) {
   if (error.IsSet())
     StopSyncing(PREFERENCES);
 }
+
+void PrefModelAssociator::SetPrefService(PrefService* pref_service) {
+  DCHECK(pref_service_ == NULL);
+  pref_service_ = pref_service;
+}