Add support for user actions to the metrics library and the metrics clients.

metrics_library.h

Index: metrics_library.h
diff --git a/metrics_library.h b/metrics_library.h
index fb31c20f6ade9df992bd8eeb1e5f8895e73eec2e..b9b817a9cf1e17f336324cbe3407d9ce2d3bb7ca 100644
--- a/metrics_library.h
+++ b/metrics_library.h
@@ -69,6 +69,15 @@ class MetricsLibrary : public MetricsLibraryInterface {
   // normal, while 100 is high).
   bool SendEnumToUMA(const std::string& name, int sample, int max);
 
+  // Sends a user action to Chrome for transport to UMA and returns true on
+  // success. This method results in the equivalent of an asynchronous
+  // non-blocking RPC to UserMetrics::RecordAction (see the comments in
+  // chrome/browser/chromeos/external_metrics.cc and
+  // chrome/browser/metrics/user_metrics.h on how to register new user actions).
+  //
+  // |action| is the user-generated event (e.g., "MuteKeyPressed").
+  bool SendUserActionToUMA(const std::string& action);
+
   // Sends to Autotest and returns true on success.
   static bool SendToAutotest(const std::string& name, int value);