Introduce a couple of abstract bases for WebDataService.

chrome/browser/api/webdata/web_data_service_base.h

Index: chrome/browser/api/webdata/web_data_service_base.h
diff --git a/chrome/browser/api/webdata/web_data_service_base.h b/chrome/browser/api/webdata/web_data_service_base.h
new file mode 100644
index 0000000000000000000000000000000000000000..43a8671833797215b634f1e340406aa234c87bfb
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+++ b/chrome/browser/api/webdata/web_data_service_base.h
@@ -0,0 +1,29 @@
+// 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;
+
+  // Returns the pointer used as a notification source for this
+  // service. This may be different than this object's |this| pointer.
+  virtual WebDataServiceBase* GetNotificationSource() = 0;

[erikwright (departed), 2012/09/13 01:45:11]

It seems you could return a NotificationSource (const-ref to member instance or
by value) and then hide the type of the source.

[Jói, 2012/09/13 12:34:50]

I think it's nicer to have it typed... if it's a NotificationSource and you need
to use the pointer, you end up having to first downcast it to
Source<WebDataServiceBase> just to get the pointer.

[erikwright (departed), 2012/09/13 13:28:10]

I don't think there's a case in this CL where the pointer is needed. It's only
used for an equality check, and the untyped NotificationSource _does_ have an
equality operator.

But I'm happy to leave it this way for now. It just seems to me that
encapsulation would be more complete if the specific type that is the source of
a notification were not leaked.

[Jói, 2012/09/13 14:48:52]

I took another look and yeah, switching to NotificationSource actually does turn
out nice and clean.  Did that.

+};
+
+#endif  // CHROME_BROWSER_API_WEBDATA_WEB_DATA_SERVICE_BASE_H_