Fix top-level trace event name in MessageLoop::RunTask

... following a typo in https://codereview.chromium.org/1237283006

BUG=523345
TBR=danakj

Review URL: https://codereview.chromium.org/1305283002

Cr-Commit-Position: refs/heads/master@{#344928}


CrOS-Libchrome-Original-Commit: b20b8680bf5a28b30149e704de43c67a0bbd3354
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 4e5594b..a44f468 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -474,7 +474,7 @@
 
   HistogramEvent(kTaskRunEvent);
 
-  TRACE_TASK_EXECUTION("toplevel", pending_task);
+  TRACE_TASK_EXECUTION("MessageLoop::RunTask", pending_task);
 
   FOR_EACH_OBSERVER(TaskObserver, task_observers_,
                     WillProcessTask(pending_task));