Fix TraceEventTestFixture flakiness

base/debug/trace_event_unittest.h

 // Copyright (c) 2012 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.
 
-#include "base/test/mock_devices_changed_observer.h"
+#include "base/time.h"
 
 namespace base {
+namespace debug {
 
-MockDevicesChangedObserver::MockDevicesChangedObserver() {
-}
+// Sleep until HighResNow has advanced by at least |elapsed|.
+void HighResSleepForTraceTest(base::TimeDelta elapsed);
 
-MockDevicesChangedObserver::~MockDevicesChangedObserver() {
-}
-
+}  // namespace debug
 }  // namespace base