mac: ensure ui::Compositor exists for visible RWHVs

ui/accelerated_widget_mac/window_resize_helper_mac.h

 // 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.
 
 #ifndef UI_ACCELERATED_WIDGET_MAC_WINDOW_RESIZE_HELPER_MAC_H_
 #define UI_ACCELERATED_WIDGET_MAC_WINDOW_RESIZE_HELPER_MAC_H_
 
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 35 matching lines @@
 class ACCELERATED_WIDGET_MAC_EXPORT WindowResizeHelperMac {
  public:
   static WindowResizeHelperMac* Get();
 
   // Initializes the pumpable task_runner(), providing it with the task runner
   // for UI thread tasks. task_runner() will be null before Init() is called,
   // and WaitForSingleTaskToRun() will immediately return false.
   void Init(
       const scoped_refptr<base::SingleThreadTaskRunner>& target_task_runner);
 
+  // Because this class is global, many tests may want to do this setup
+  // repeatedly, so need some way to uninitialize as well.
+  void ShutdownForTests();
+
   scoped_refptr<base::SingleThreadTaskRunner> task_runner() const;
 
   // UI THREAD ONLY -----------------------------------------------------------
 
   // Waits at most |max_delay| for a task to run. Returns true if a task ran,
   // false if no task ran.
   bool WaitForSingleTaskToRun(const base::TimeDelta& max_delay);
 
  private:
   friend struct base::DefaultLazyInstanceTraits<WindowResizeHelperMac>;
   WindowResizeHelperMac();
   ~WindowResizeHelperMac();
 
   // This helper is needed to create a ScopedAllowWait inside the scope of a
   // class where it is allowed.
   static void EventTimedWait(base::WaitableEvent* event, base::TimeDelta delay);
 
   // The task runner to which the helper will post tasks. This also maintains
   // the task queue and does the actual work for WaitForSingleTaskToRun.
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowResizeHelperMac);
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCELERATED_WIDGET_MAC_WINDOW_RESIZE_HELPER_MAC_H_