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

ash/wm/root_window_layout_manager_unittest.cc

Index: ash/wm/root_window_layout_manager_unittest.cc
diff --git a/ash/wm/root_window_layout_manager_unittest.cc b/ash/wm/root_window_layout_manager_unittest.cc
new file mode 100644
index 0000000000000000000000000000000000000000..57693016801cfe9846739accad0caee880aa27b6
--- /dev/null
+++ b/ash/wm/root_window_layout_manager_unittest.cc
@@ -0,0 +1,46 @@
+// 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 "ash/shell.h"
+#include "ash/shell_window_ids.h"
+#include "ash/test/ash_test_base.h"
+#include "ui/aura/test/test_windows.h"
+#include "ui/aura/window_observer.h"
+
+namespace ash {
+namespace {
+
+class WindowDeleter : public aura::WindowObserver {
+ public:
+  explicit WindowDeleter(aura::Window* window) : target_(window) {}
+
+  // aura::WindowObserver::
+  void OnWindowBoundsChanged(aura::Window* window,
+                             const gfx::Rect& old_bounds,
+                             const gfx::Rect& new_bounds) override {
+    delete target_;
+  }
+
+ private:
+  aura::Window* target_;
+
+  DISALLOW_COPY_AND_ASSIGN(WindowDeleter);
+};
+
+}  // namespace
+
+using RootWindowLayoutManagerTest = test::AshTestBase;
+
+TEST_F(RootWindowLayoutManagerTest, DeleteChildDuringResize) {
+  aura::Window* parent = Shell::GetPrimaryRootWindow()->GetChildById(
+      kShellWindowId_DesktopBackgroundContainer);
+  aura::Window* w1 = aura::test::CreateTestWindowWithId(1, parent);
+  aura::Window* w2 = aura::test::CreateTestWindowWithId(2, parent);
+  WindowDeleter deleter(w1);
+  w2->AddObserver(&deleter);
+  UpdateDisplay("600x500");
+  w2->RemoveObserver(&deleter);
+}
+
+}  // namespace ash