Add histograms to track synced pref changes.

chrome/browser/prefs/pref_service_syncable.h

Index: chrome/browser/prefs/pref_service_syncable.h
diff --git a/chrome/browser/prefs/pref_service_syncable.h b/chrome/browser/prefs/pref_service_syncable.h
index 4f4715e8c881488505bdc138b81338e3f15f9968..b8390f1b273a27213d8af3f7d134a9f293eeac88 100644
--- a/chrome/browser/prefs/pref_service_syncable.h
+++ b/chrome/browser/prefs/pref_service_syncable.h
@@ -7,6 +7,7 @@
 
 #include "base/prefs/pref_service.h"
 #include "chrome/browser/prefs/pref_model_associator.h"
+#include "chrome/browser/prefs/synced_pref_observer.h"
 #include "components/user_prefs/pref_registry_syncable.h"
 
 class PrefServiceSyncableObserver;
@@ -80,6 +81,11 @@ class PrefServiceSyncable : public PrefService {
   // Do not call this after having derived an incognito or per tab pref service.
   virtual void UpdateCommandLinePrefStore(PrefStore* cmd_line_store) OVERRIDE;
 
+  void AddSyncedPrefObserver(const std::string& name,
+                             SyncedPrefObserver* observer);
+  void RemoveSyncedPrefObserver(const std::string& name,
+                                SyncedPrefObserver* observer);
+
  private:
   friend class PrefModelAssociator;