Add LayoutTests for the fixed multithread GC bug of ServiceWorker Navigation Preload

third_party/WebKit/Source/core/testing/WorkerInternals.idl

 // Copyright 2016 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.
 
 interface WorkerInternals {
     OriginTrialsTest originTrialsTest();
 
     // Records |feature| in worker's use counter. |feature| must be one of the
     // values from the UseCounter::Feature enum.
     [CallWith=ScriptState] void countFeature(unsigned long feature);
     [CallWith=ScriptState] void countDeprecation(unsigned long feature);
+
+    [CallWith=ScriptState] void collectGarbage();
 };