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.
+// This function must be called after the task runner has been set with
+// SetRecordActionTaskRunner().
 //
 // "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
 // with its metric name. Therefore, it will be possible to retrieve the metric
 // name from the hash later on.
 //
 // Once a new recorded action is added, run
 //   tools/metrics/actions/extract_actions.py
 // to add the metric to actions.xml, then update the <owner>s and <description>
 // sections. Make sure to include the actions.xml file when you upload your code
 // for review!
 //
 // For more complicated situations (like when there are many different
-// possible actions), see RecordComputedAction.
+// possible actions), see RecordComputedAction().
 BASE_EXPORT void RecordAction(const UserMetricsAction& action);
 
-// This function has identical input and behavior to RecordAction, but is
+// This function has identical input and behavior to RecordAction(), but is
 // not automatically found by the action-processing scripts.  It can be used
 // when it's a pain to enumerate all possible actions, but if you use this
 // you need to also update the rules for extracting known actions in
 // tools/metrics/actions/extract_actions.py.
+// This function must be called after the task runner has been set with
+// SetRecordActionTaskRunner().
 BASE_EXPORT void RecordComputedAction(const std::string& action);
 
 // Called with the action string.
-typedef base::Callback<void(const std::string&)> ActionCallback;
+typedef Callback<void(const std::string&)> ActionCallback;
 
 // Add/remove action callbacks (see above).
+// These functions must be called after the task runner has been set with
+// SetRecordActionTaskRunner().
 BASE_EXPORT void AddActionCallback(const ActionCallback& callback);
 BASE_EXPORT void RemoveActionCallback(const ActionCallback& callback);
 
+// Set the task runner on which to record actions.
+BASE_EXPORT void SetRecordActionTaskRunner(
+    scoped_refptr<SingleThreadTaskRunner> task_runner);
+
 }  // namespace base
 
 #endif  // BASE_METRICS_USER_METRICS_H_