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..1bf2f1ce5ca5e56f2d9f16f26ec89bdfc2d93a9a 100644
--- a/content/public/browser/user_metrics.h
+++ b/content/public/browser/user_metrics.h
@@ -7,6 +7,8 @@
 
 #include <string>
 
+#include "base/callback.h"
+#include "base/threading/non_thread_safe.h"
 #include "content/common/content_export.h"
 
 namespace content {
@@ -50,6 +52,13 @@ CONTENT_EXPORT void RecordAction(const UserMetricsAction& action);
 // chrome/tools/extract_actions.py.
 CONTENT_EXPORT void RecordComputedAction(const std::string& action);
 
+// Called with the action string.
+typedef base::Callback<void(const std::string&)> ActionCallback;
+
+// Add/remove action callbacks (see above).
+CONTENT_EXPORT void AddActionCallback(const ActionCallback& callback);
+CONTENT_EXPORT void RemoveActionCallback(const ActionCallback& callback);
+
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_USER_METRICS_H_