Introduce a couple of abstract bases for WebDataService.

chrome/browser/api/webdata/web_data_service_base.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_API_WEBDATA_WEB_DATA_SERVICE_BASE_H_
+#define CHROME_BROWSER_API_WEBDATA_WEB_DATA_SERVICE_BASE_H_
+
+#include "chrome/browser/profiles/refcounted_profile_keyed_service.h"
+
+class Profile;
+
+// Base for WebDataService class hierarchy.
+class WebDataServiceBase {
+ public:
+  // All requests return an opaque handle of the following type.
+  typedef int Handle;
+
+  virtual ~WebDataServiceBase() {}
+
+  // Cancel any pending request. You need to call this method if your
+  // WebDataServiceConsumer is about to be deleted.
+  virtual void CancelRequest(Handle h) = 0;
+};
+
+#endif  // CHROME_BROWSER_API_WEBDATA_WEB_DATA_SERVICE_BASE_H_