Move the thread hop for UMA user actions from content:: to base::.

base/metrics/user_metrics.h

Index: base/metrics/user_metrics.h
diff --git a/base/metrics/user_metrics.h b/base/metrics/user_metrics.h
index bcfefb809942e203590dfd9db7672cc56f861c7c..acfecd696dc69ac06e6a023990e78ee3b6af3d4b 100644
--- a/base/metrics/user_metrics.h
+++ b/base/metrics/user_metrics.h
@@ -10,6 +10,7 @@
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/metrics/user_metrics_action.h"
+#include "base/single_thread_task_runner.h"
 
 namespace base {
 
@@ -55,6 +56,11 @@ typedef base::Callback<void(const std::string&)> ActionCallback;
 BASE_EXPORT void AddActionCallback(const ActionCallback& callback);
 BASE_EXPORT void RemoveActionCallback(const ActionCallback& callback);
 
+// Set the task runner on which to record actions. The task runner can only be
+// set once, unless ResetRecordActionTaskRunnerForTesting is called.
+BASE_EXPORT void SetRecordActionTaskRunner(
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+
 }  // namespace base
 
 #endif  // BASE_METRICS_USER_METRICS_H_