content: convert user action notification to observer usage

content/public/browser/user_metrics.h

Index: content/public/browser/user_metrics.h
diff --git a/content/public/browser/user_metrics.h b/content/public/browser/user_metrics.h
index d1f39bfb63d9cb20afd9497b471f08b0b70e720c..931f78b37cca5d25b45d0752d8ec6ce648b9f817 100644
--- a/content/public/browser/user_metrics.h
+++ b/content/public/browser/user_metrics.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/threading/non_thread_safe.h"
 #include "content/common/content_export.h"
 
 namespace content {
@@ -50,6 +51,21 @@ CONTENT_EXPORT void RecordAction(const UserMetricsAction& action);
 // 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_