Use WindowTracker when resizing root window as it may delete other window in unit tests.

ui/aura/window_tracker.cc

 // 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 "ui/aura/window_tracker.h"
 
 #include "ui/aura/window.h"
 
 namespace aura {
 
 WindowTracker::WindowTracker() {
 }
 
+WindowTracker::WindowTracker(const WindowList& windows) {
+  // |windows| may contain dups, so call Add() instead of insert().
+  for (auto iter = windows.begin(); iter != windows.end(); iter++)
+    Add(*iter);
+}
+
 WindowTracker::~WindowTracker() {
-  for (Windows::iterator i = windows_.begin(); i != windows_.end(); ++i)
-    (*i)->RemoveObserver(this);
+  while (has_windows())
+    Pop()->RemoveObserver(this);
 }
 
 void WindowTracker::Add(Window* window) {
   if (windows_.count(window))
     return;
 
   window->AddObserver(this);
   windows_.insert(window);
 }
 
 void WindowTracker::Remove(Window* window) {
   if (windows_.count(window)) {
     windows_.erase(window);
     window->RemoveObserver(this);
   }
 }
 
 bool WindowTracker::Contains(Window* window) {
   return windows_.count(window) > 0;
 }
 
+aura::Window* WindowTracker::Pop() {
+  DCHECK(!windows_.empty());
+  aura::Window* window = *windows_.begin();
+  Remove(window);
+  return window;
+}
+
 void WindowTracker::OnWindowDestroying(Window* window) {
   DCHECK_GT(windows_.count(window), 0u);
   Remove(window);
 }
 
 }  // namespace aura