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

c_metrics_library.h

Index: c_metrics_library.h
diff --git a/c_metrics_library.h b/c_metrics_library.h
index e691ad66ef7c4bfe7c962785ec241593d02d65fc..9500aade5d003234f3768a081dc3191c8c941ebf 100644
--- a/c_metrics_library.h
+++ b/c_metrics_library.h
@@ -28,6 +28,10 @@ int CMetricsLibrarySendToUMA(CMetricsLibrary handle,
 int CMetricsLibrarySendEnumToUMA(CMetricsLibrary handle,
                                  const char* name, int sample, int max);
 
+// C wrapper for MetricsLibrary::SendUserActionToUMA.
+int CMetricsLibrarySendUserActionToUMA(CMetricsLibrary handle,
+                                       const char* action);
+
 // C wrapper for MetricsLibrary::AreMetricsEnabled.
 int CMetricsLibraryAreMetricsEnabled(CMetricsLibrary handle);