Fix DirectCompositionSurfaceTest.TestMakeCurrent flakiness.

gpu/ipc/service/child_window_win.cc

 // Copyright 2017 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 "gpu/ipc/service/child_window_win.h"
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/memory/ptr_util.h"
@@ skipping 189 matching lines @@
   if (shared_data_) {
     scoped_refptr<base::TaskRunner> task_runner =
         shared_data_->thread.task_runner();
     task_runner->PostTaskAndReply(
         FROM_HERE,
         base::Bind(&DestroyWindowsOnThread, window_, initial_parent_window_),
         base::Bind(&DestroySharedData, base::Passed(std::move(shared_data_))));
   }
 }
 
+scoped_refptr<base::TaskRunner> ChildWindowWin::GetTaskRunnerForTesting() {
+  DCHECK(shared_data_);
+  return shared_data_->thread.task_runner();
+}
+
 }  // namespace gpu