[UMA] Fix a crash on logging user actions posted from a non-UI thread.

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"
 
 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.
+//
 // "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
@@ skipping 19 matching lines @@
 // 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);
 
 }  // namespace base
 
 #endif  // BASE_METRICS_USER_METRICS_H_