Create an AutofillBackend interface

components/autofill/browser/webdata/autofill_webdata_service.h

Index: components/autofill/browser/webdata/autofill_webdata_service.h
diff --git a/components/autofill/browser/webdata/autofill_webdata_service.h b/components/autofill/browser/webdata/autofill_webdata_service.h
index 87b967ca94bb2283270198968fe60b21399cfbf4..a31bb8e6747b952ff1fa511000cebfb55dd5ec9f 100644
--- a/components/autofill/browser/webdata/autofill_webdata_service.h
+++ b/components/autofill/browser/webdata/autofill_webdata_service.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/supports_user_data.h"
 #include "components/autofill/browser/webdata/autofill_webdata.h"
@@ -31,6 +32,7 @@ class AutofillWebDataBackend;
 class AutofillWebDataServiceObserverOnDBThread;
 class AutofillWebDataServiceObserverOnUIThread;
 class CreditCard;
+class SyncableServiceBackend;
 
 // API for Autofill web data.
 class AutofillWebDataService : public AutofillWebData,
@@ -96,11 +98,21 @@ class AutofillWebDataService : public AutofillWebData,
   // |ShutdownOnUIThread()| is called.
   base::SupportsUserData* GetDBUserData();
 
+  // Takes a callback which will be called on the DBThread with a pointer to a

[Ilya Sherman, 2013/05/08 00:39:06]

nit: "DBThread" -> "DB thread"; ditto below.

[Cait (Slow), 2013/05/08 19:16:01]

Done.

+  // |SyncableServiceBackend|. This backend can be used to access or update the
+  // WebDatabase directly on the DBThread.
+  void GetSyncableBackend(
+      const base::Callback<void(SyncableServiceBackend*)>& callback);

[Ilya Sherman, 2013/05/08 00:39:06]

Should this be returned as a scoped_refptr?  What's guaranteeing the lifetime of
the syncable backend for the client?

[Cait (Slow), 2013/05/08 19:16:01]

Both the backend and the syncable services are owned by the
AutofillWebDataService. The backend will outlive the syncable services as they
(the syncable services) get destroyed on the first pass of the shutdown process
(ShutdownOnUIThread) and the backend does not get destroyed until the second
pass (dtor). 

+
  protected:
   virtual ~AutofillWebDataService();
 
   virtual void ShutdownOnDBThread();
 
+  // This factory is used on the UIThread. All vended weak pointers are

[Ilya Sherman, 2013/05/08 00:39:06]

nit: "UIThread" -> "UI thread"

[Cait (Slow), 2013/05/08 19:16:01]

Done.

+  // invalidated in ShutdownOnUIThread().
+  base::WeakPtrFactory<AutofillWebDataService> weak_ptr_factory_;

[Ilya Sherman, 2013/05/08 00:39:06]

nit: Member variables should always have private visibility.

[Cait (Slow), 2013/05/08 19:16:01]

Done.

+
  private:
   // This makes the destructor public, and thus allows us to aggregate
   // SupportsUserData. It is private by default to prevent incorrect