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..fe51f800f679ea8be1b9d24bb8ce93cb5e6c73a8 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"
@@ -31,11 +32,15 @@ class AutofillWebDataBackend;
 class AutofillWebDataServiceObserverOnDBThread;
 class AutofillWebDataServiceObserverOnUIThread;
 class CreditCard;
+class SyncableServiceDelegate;
 
 // API for Autofill web data.
 class AutofillWebDataService : public AutofillWebData,
                                public WebDataServiceBase {
  public:
+  typedef base::Callback<void(base::WeakPtr<SyncableServiceDelegate>)>
+      DelegateOnDBCallback;
+
   AutofillWebDataService();
 
   AutofillWebDataService(scoped_refptr<WebDatabaseService> wdbs,
@@ -96,11 +101,21 @@ class AutofillWebDataService : public AutofillWebData,
   // |ShutdownOnUIThread()| is called.
   base::SupportsUserData* GetDBUserData();
 
+  // Asynchronously gets an instance of |AutofillBackendDelegate|, which can be
+  // used to do work on the DB thread. This method is meant to be called from
+  // the UI thread, but the callback will be called on the DB thread.
+  // The delegate instance is owned by |AutofillWebDataBackend|, and so has the
+  // same lifetime as |AutofillWebDataService|.
+  void GetDelegateOnDB(const DelegateOnDBCallback& del_callback);

[Ilya Sherman, 2013/05/07 00:02:43]

nit: Please don't use abbreviations like "del" in names.

+
  protected:
   virtual ~AutofillWebDataService();
 
   virtual void ShutdownOnDBThread();
 
+  // All vended weak pointers are invalidated in ShutdownDatabase().

[Jói, 2013/05/06 23:08:39]

Suggest documenting which thread this factory is used from, since it can only be
used from one.

[Cait (Slow), 2013/05/07 19:22:07]

Done.

+  base::WeakPtrFactory<AutofillWebDataService> weak_ptr_factory_;
+
  private:
   // This makes the destructor public, and thus allows us to aggregate
   // SupportsUserData. It is private by default to prevent incorrect