Add callback list to PrefModelAssociator after sync data is loaded

components/content_settings/core/browser/host_content_settings_map.h

Index: components/content_settings/core/browser/host_content_settings_map.h
diff --git a/components/content_settings/core/browser/host_content_settings_map.h b/components/content_settings/core/browser/host_content_settings_map.h
index b5af0111f8c063d2fd5a7d9cbb16530903957f91..7d6a4774ed16fdbd38b15e2aab61cb1aafc3c89d 100644
--- a/components/content_settings/core/browser/host_content_settings_map.h
+++ b/components/content_settings/core/browser/host_content_settings_map.h
@@ -24,6 +24,7 @@
 #include "components/content_settings/core/common/content_settings_types.h"
 #include "components/keyed_service/core/refcounted_keyed_service.h"
 #include "components/prefs/pref_change_registrar.h"
+#include "components/syncable_prefs/pref_service_syncable.h"
 
 class ExtensionService;
 class GURL;
@@ -41,6 +42,10 @@ class PrefProvider;
 class TestUtils;
 }
 
+namespace syncable_prefs {
+class PrefServiceSyncable;
+}
+
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
@@ -66,7 +71,8 @@ class HostContentSettingsMap : public content_settings::Observer,
   // |is_incognito_profile| and |is_guest_profile| should be true.
   HostContentSettingsMap(PrefService* prefs,
                          bool is_incognito_profile,
-                         bool is_guest_profile);
+                         bool is_guest_profile,
+                         syncable_prefs::PrefServiceSyncable* pref_service);

[raymes, 2016/06/29 03:50:01]

I'm wondering if we can just pass in a base::Callback here
register_pref_sync_finished_callback

register_pref_sync_finished_callback.Run(base::Bind(...));

That would eliminate the dependency on syncable_prefs and make testing a lot
easier.

We could even register the callback outside this class (without passing anything
in), but I think this is slightly better because it means that the client code
is forced to think about registering such a callback. What do you think?

[lshang, 2016/06/30 05:03:36]

Done. 
Yeah as we discussed, I think register the callback outside this class sounds
better because that will make the constructor of HostContentSettingsMap cleaner.
I've moved it to HostContentSettingsMapFactory. And I also commented out the
registration code block for now since we don't really want it to be called in
this CL. we should hook all things up in the next CL when we change the scoping
type.

 
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
 
@@ -396,6 +402,8 @@ class HostContentSettingsMap : public content_settings::Observer,
 
   base::ObserverList<content_settings::Observer> observers_;
 
+  syncable_prefs::PrefServiceSyncable* pref_service_;
+
   DISALLOW_COPY_AND_ASSIGN(HostContentSettingsMap);
 };