Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc

Index: chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc
diff --git a/chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc b/chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc
index 557b26d7819e6115eac26685b0647bac4d53f660..a465a1681d55eeb07852ef4b3a9295dededb872e 100644
--- a/chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc
+++ b/chrome/browser/ui/ash/launcher/chrome_launcher_controller_per_app.cc
@@ -19,7 +19,7 @@
 #include "chrome/browser/extensions/extension_system.h"
 #include "chrome/browser/favicon/favicon_tab_helper.h"
 #include "chrome/browser/prefs/incognito_mode_prefs.h"
-#include "chrome/browser/prefs/pref_service.h"
+#include "chrome/browser/prefs/pref_service_syncable.h"
 #include "chrome/browser/prefs/scoped_user_pref_update.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
@@ -220,7 +220,7 @@ ChromeLauncherControllerPerApp::~ChromeLauncherControllerPerApp() {
   if (app_sync_ui_state_)
     app_sync_ui_state_->RemoveObserver(this);
 
-  profile_->GetPrefs()->RemoveObserver(this);
+  PrefServiceSyncable::FromProfile(profile_)->RemoveObserver(this);

[Mattias Nissler (ping if slow), 2013/02/06 17:53:33]

Oh, this is the OnIsSyncingChanged stuff. I'll have to look at that separately
to find out why the hell we have that, no need to wait for that.

[Jói, 2013/02/07 14:52:32]

OK.

[Mattias Nissler (ping if slow), 2013/02/08 14:49:18]

FWIW, I've looked into this some more. I've found that the IsSyncing is used for
the following: Waiting for the sync value of a pref to arrive and then not
applying it blindly but making the decision for what pref value to use actually
based on additional factors. This pattern is present in both
MaybePropagatePrefToLocal in chrome_launcher_controller_per_{browser,app}.cc and
chromeos::Preferences::ForceNaturalScrollDefault() (which seem to be the only
consumers of this).

I think we could cover this need nicely way by allowing custom pref merge logic
for synced-in data. Now that we have PrefRegistrySyncable, that should be
relatively simple by adding something like this:

typedef base::Callback<scoped_ptr<Value>(const Value* local_value, const Value*
sync_value)) MergeCallback;
void PrefRegistrySyncable::RegisterMergeCallback(const char* pref_name, const
MergeCallback& callback);

This would also allow us to get rid of the duplicated prefs used by
ChromeLauncherController.

 }
 
 void ChromeLauncherControllerPerApp::Init() {
@@ -230,7 +230,7 @@ void ChromeLauncherControllerPerApp::Init() {
   if (ash::Shell::HasInstance()) {
     SetShelfAutoHideBehaviorFromPrefs();
     SetShelfAlignmentFromPrefs();
-    PrefServiceSyncable* prefs = profile_->GetPrefs();
+    PrefServiceSyncable* prefs = PrefServiceSyncable::FromProfile(profile_);
     if (!prefs->FindPreference(prefs::kShelfAlignmentLocal)->HasUserSetting() ||
         !prefs->FindPreference(prefs::kShelfAutoHideBehaviorLocal)->
             HasUserSetting()) {
@@ -937,10 +937,11 @@ void ChromeLauncherControllerPerApp::OnShelfAlignmentChanged(
 }
 
 void ChromeLauncherControllerPerApp::OnIsSyncingChanged() {
-  MaybePropagatePrefToLocal(profile_->GetPrefs(),
+  PrefServiceSyncable* prefs = PrefServiceSyncable::FromProfile(profile_);
+  MaybePropagatePrefToLocal(prefs,
                             prefs::kShelfAlignmentLocal,
                             prefs::kShelfAlignment);
-  MaybePropagatePrefToLocal(profile_->GetPrefs(),
+  MaybePropagatePrefToLocal(prefs,
                             prefs::kShelfAutoHideBehaviorLocal,
                             prefs::kShelfAutoHideBehavior);
 }