Fix nits on metrics_services_manager_client.h

components/metrics_services_manager/metrics_services_manager_client.h

Index: components/metrics_services_manager/metrics_services_manager_client.h
diff --git a/components/metrics_services_manager/metrics_services_manager_client.h b/components/metrics_services_manager/metrics_services_manager_client.h
index 9a404865c3c958aa69f6c0ea5bc9e474155e52a7..eabc53baa89fb69831623e3553c9f0e12599d07c 100644
--- a/components/metrics_services_manager/metrics_services_manager_client.h
+++ b/components/metrics_services_manager/metrics_services_manager_client.h
@@ -5,12 +5,10 @@
 #ifndef COMPONENTS_METRICS_SERVICES_MANAGER_METRICS_SERVICES_MANAGER_CLIENT_H_
 #define COMPONENTS_METRICS_SERVICES_MANAGER_METRICS_SERVICES_MANAGER_CLIENT_H_
 
-#include "base/callback.h"
+#include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/threading/thread_checker.h"
 
 namespace metrics {
-class MetricsService;
 class MetricsServiceClient;
 }