Introduce per-tab preferences service.

chrome/browser/prefs/pref_model_associator.h

Index: chrome/browser/prefs/pref_model_associator.h
diff --git a/chrome/browser/prefs/pref_model_associator.h b/chrome/browser/prefs/pref_model_associator.h
index 5e2189cfc375486907207b86f9cb3d8ccb15b722..cf1dd77456d653841425c69c34b5c3dc034af242 100644
--- a/chrome/browser/prefs/pref_model_associator.h
+++ b/chrome/browser/prefs/pref_model_associator.h
@@ -32,7 +32,7 @@ class PrefModelAssociator
     : public SyncableService,
       public base::NonThreadSafe {
  public:
-  explicit PrefModelAssociator(PrefService* pref_service);
+  PrefModelAssociator();
   virtual ~PrefModelAssociator();
 
   // SyncableService implementation.
@@ -64,6 +64,8 @@ class PrefModelAssociator
   // sent to the syncer.
   virtual void ProcessPrefChange(const std::string& name);
 
+  void SetPrefService(PrefService* pref_service);
+
   // Merges the value of local_pref into the supplied server_value and returns
   // the result (caller takes ownership). If there is a conflict, the server
   // value always takes precedence. Note that only certain preferences will
@@ -89,9 +91,6 @@ class PrefModelAssociator
 
   typedef std::map<std::string, SyncData> SyncDataMap;
 
-  // For testing.
-  PrefModelAssociator();
-
   // Create an association for a given preference. If |sync_pref| is valid,
   // signifying that sync has data for this preference, we reconcile their data
   // with ours and append a new UPDATE SyncChange to |sync_changes|. If