mac: ensure ui::Compositor exists for visible RWHVs

ui/accelerated_widget_mac/window_resize_helper_mac.cc

 // Copyright 2015 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 "ui/accelerated_widget_mac/window_resize_helper_mac.h"
 
 #include <stdint.h>
 
 #include <list>
 
@@ skipping 283 matching lines @@
   return g_window_resize_helper.Pointer();
 }
 
 void WindowResizeHelperMac::Init(
     const scoped_refptr<base::SingleThreadTaskRunner>& target_task_runner) {
   DCHECK(!task_runner_);
   task_runner_ = new PumpableTaskRunner(
       base::Bind(&WindowResizeHelperMac::EventTimedWait), target_task_runner);
 }
 
+void WindowResizeHelperMac::ShutdownForTests() {
+  task_runner_ = nullptr;
+}
+
 bool WindowResizeHelperMac::WaitForSingleTaskToRun(
     const base::TimeDelta& max_delay) {
   PumpableTaskRunner* pumpable_task_runner =
       static_cast<PumpableTaskRunner*>(task_runner_.get());
   if (!pumpable_task_runner)
     return false;
   return pumpable_task_runner->WaitForSingleWrappedTaskToRun(max_delay);
 }
 
 WindowResizeHelperMac::WindowResizeHelperMac() {}
 WindowResizeHelperMac::~WindowResizeHelperMac() {}
 
 void WindowResizeHelperMac::EventTimedWait(base::WaitableEvent* event,
                                            base::TimeDelta delay) {
   base::ThreadRestrictions::ScopedAllowWait allow_wait;
   event->TimedWait(delay);
 }
 
 }  // namespace ui