Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/browser/profiles/profile_keyed_base_factory.cc

Index: chrome/browser/profiles/profile_keyed_base_factory.cc
diff --git a/chrome/browser/profiles/profile_keyed_base_factory.cc b/chrome/browser/profiles/profile_keyed_base_factory.cc
index 1a38c08f2d7a57ef2fcaf34413baddd52ffaf6f8..858cdadf1e0dd1e3fe823e312d16a8fa8888d644 100644
--- a/chrome/browser/profiles/profile_keyed_base_factory.cc
+++ b/chrome/browser/profiles/profile_keyed_base_factory.cc
@@ -4,6 +4,8 @@
 
 #include "chrome/browser/profiles/profile_keyed_base_factory.h"
 
+#include "chrome/browser/prefs/pref_registry_syncable.h"
+#include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_dependency_manager.h"
 
@@ -76,7 +78,13 @@ void ProfileKeyedBaseFactory::RegisterUserPrefsOnProfile(Profile* profile) {
 
   std::set<Profile*>::iterator it = registered_preferences_.find(profile);
   if (it == registered_preferences_.end()) {
-    RegisterUserPrefs(profile->GetPrefs());
+    PrefService* prefs = profile->GetPrefs();
+    PrefRegistrySyncable* registry = static_cast<PrefRegistrySyncable*>(
+        prefs->DeprecatedGetPrefRegistry());
+    RegisterUserPrefs(registry);
+    // A few registration functions still need the PrefService pointer
+    // (e.g. to clear preferences).
+    DeprecatedRegisterUserPrefs(prefs, registry);

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

Yikes, hacks over hacks here. I'll have to think a bit more about registrations
coming from PKSs.

[Jói, 2013/02/07 14:52:32]

Yeah, when I ran into this I thought "oops" :)  But better to clean such things
up in separate smaller changes I think.   

I figured that for now, we'd be better off with a separate DeprecatedXyz method
that gets the PrefService parameter, rather than passing PrefService in the main
method, so that people don't start accidentally introducing more dependencies.

[Mattias Nissler (ping if slow), 2013/02/08 11:26:56]

So, I wonder whether we'd be better off if we just had all the PKSF that are
compiled in register their prefs when a profile initializes. Admittedly this
will register prefs that are not used (because the corresponding PKS is not
instantiated), but that's a price I'd be willing to pay.

     registered_preferences_.insert(profile);
   }
 }