content: convert user action notification to observer usage

content/public/browser/user_metrics.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 CONTENT_PUBLIC_BROWSER_USER_METRICS_H_
 #define CONTENT_PUBLIC_BROWSER_USER_METRICS_H_
 
 #include <string>
 
+#include "base/threading/non_thread_safe.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 // This module provides some helper functions for logging actions tracked by
 // the user metrics system.
 
 
 // UserMetricsAction exist purely to standardize on the paramters passed to
 // UserMetrics. That way, our toolset can scan the sourcecode reliable for
@@ skipping 23 matching lines @@
 // possible actions), see RecordComputedAction.
 CONTENT_EXPORT void RecordAction(const UserMetricsAction& action);
 
 // This function has identical input and behavior to RecordAction, but is
 // not automatically found by the action-processing scripts.  It can be used
 // when it's a pain to enumerate all possible actions, but if you use this
 // you need to also update the rules for extracting known actions in
 // chrome/tools/extract_actions.py.
 CONTENT_EXPORT void RecordComputedAction(const std::string& action);
 
+class UserMetricsObserver : public base::NonThreadSafe {
+ public:
+  // Called when the user does a gesture resulting in a noteworthy action
+  // taking place.
+  virtual void UserAction(const std::string& action);

[jam, 2013/01/29 23:53:25]

since this is just one method on the interface, a method to add and remove a
callback would be enough as well

+
+ protected:
+  // Constructor and destructor automatically handle registration
+  // and deregistration of the observer.
+  UserMetricsObserver();
+  virtual ~UserMetricsObserver();
+
+  DISALLOW_COPY_AND_ASSIGN(UserMetricsObserver);
+};
+
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_USER_METRICS_H_