Create an AutofillBackend interface

components/webdata/common/web_database_service.h

Index: components/webdata/common/web_database_service.h
diff --git a/components/webdata/common/web_database_service.h b/components/webdata/common/web_database_service.h
index 1bdfca8504edc317b12d9360a1082f07262fc8f3..6ba498ec49c0bd768c6d79743d8e73e521ef2759 100644
--- a/components/webdata/common/web_database_service.h
+++ b/components/webdata/common/web_database_service.h
@@ -51,6 +51,7 @@ class WEBDATA_EXPORT WebDatabaseService
  public:
   typedef base::Callback<scoped_ptr<WDTypedResult>(WebDatabase*)> ReadTask;
   typedef base::Callback<WebDatabase::State(WebDatabase*)> WriteTask;
+  typedef base::Callback<void(WebDatabase*)> DBCallback;
 
   // Takes the path to the WebDatabase file.
   explicit WebDatabaseService(const base::FilePath& path);
@@ -75,6 +76,11 @@ class WEBDATA_EXPORT WebDatabaseService
   // TODO(caitkp): remove this method once SyncServices no longer depend on it.
   virtual WebDatabase* GetDatabaseOnDB() const;
 
+  // Asynchronously gets an instance of |WebDatabase|, 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.
+  void GetDatabaseOnDB(const DBCallback& callback);

[Jói, 2013/05/06 23:08:39]

I'm wondering about naming here. The name could easily be misunderstood to mean
you should call the method from the DB thread... do you know if there is
precedent for this kind of name?

+
   // Schedule an update/write task on the DB thread.
   virtual void ScheduleDBTask(
       const tracked_objects::Location& from_here,