Add histograms to track synced pref changes.

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 584e27172252a7b7853654bc33d9fe3f1e6f9e28..539d38a301c9b71d15547053c8c8093cd5268cd0 100644
--- a/chrome/browser/prefs/pref_model_associator.h
+++ b/chrome/browser/prefs/pref_model_associator.h
@@ -11,7 +11,10 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/containers/hash_tables.h"
+#include "base/observer_list.h"
 #include "base/threading/non_thread_safe.h"
+#include "chrome/browser/prefs/synced_pref_observer.h"
 #include "sync/api/sync_data.h"
 #include "sync/api/syncable_service.h"
 
@@ -97,6 +100,14 @@ class PrefModelAssociator
   // Note this does not refer to SYNCABLE_PREF.
   bool IsPrefSynced(const std::string& name) const;
 
+  // Adds a SyncedPrefObserver to watch for changes to a specific pref.
+  void AddSyncedPrefObserver(const std::string& name,
+                             SyncedPrefObserver* observer);
+
+  // Removes a SyncedPrefObserver from a pref's list of observers.
+  void RemoveSyncedPrefObserver(const std::string& name,
+                                SyncedPrefObserver* observer);
+
  protected:
   friend class ProfileSyncServicePreferenceTest;
 
@@ -157,6 +168,18 @@ class PrefModelAssociator
   // PRIORITY_PREFERENCES.
   syncer::ModelType type_;
 
+ private:
+  // Map prefs to lists of observers. Observers will receive notification when
+  // a pref changes, including the detail of whether or not the change came
+  // from sync.
+  typedef ObserverList<SyncedPrefObserver> SyncedPrefObserverList;
+  typedef base::hash_map<std::string, SyncedPrefObserverList*>
+      SyncedPrefObserverMap;
+
+  void NotifySyncedPrefObservers(const std::string& path, bool from_sync) const;
+
+  SyncedPrefObserverMap synced_pref_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(PrefModelAssociator);
 };