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

base/metrics/user_metrics.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_METRICS_USER_METRICS_H_
 #define BASE_METRICS_USER_METRICS_H_
 
 #include <string>
 
 #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 {
 
 // This module provides some helper functions for logging actions tracked by
 // the user metrics system.
 
 // Record that the user performed an action.
 // This method *must* be called from the main thread.

[Lei Zhang, 2016/04/18 21:34:13]

Does this need updating?

[beaudoin, 2016/04/19 16:57:27]

Done.

 //
 // "Action" here means a user-generated event:
 //   good: "Reload", "CloseTab", and "IMEInvoked"
 //   not good: "SSLDialogShown", "PageLoaded", "DiskFull"
 // We use this to gather anonymized information about how users are
 // interacting with the browser.
 // WARNING: In calls to this function, UserMetricsAction and a
 // string literal parameter must be on the same line, e.g.
 //   RecordAction(UserMetricsAction("my extremely long action name"));
 // This ensures that our processing scripts can associate this action's hash
@@ skipping 17 matching lines @@
 // tools/metrics/actions/extract_actions.py.
 BASE_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).
 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.

[sgurun-gerrit only, 2016/04/19 15:36:06]

is it optional to call this? if not called what is the default behavior.

[beaudoin, 2016/04/19 16:57:27]

Must be called before any of the other functions here. Added a comment to
clarify this.

+BASE_EXPORT void SetRecordActionTaskRunner(
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+
 }  // namespace base
 
 #endif  // BASE_METRICS_USER_METRICS_H_