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

ui/aura/window_tracker.h

Index: ui/aura/window_tracker.h
diff --git a/ui/aura/window_tracker.h b/ui/aura/window_tracker.h
index cd839697a0f3c87694746f13e9b8b02ed7691231..d666c2b186978e2490c49702da5cba5778c30afb 100644
--- a/ui/aura/window_tracker.h
+++ b/ui/aura/window_tracker.h
@@ -6,6 +6,7 @@
 #define UI_AURA_WINDOW_TRACKER_H_
 
 #include <set>
+#include <vector>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
@@ -15,14 +16,18 @@
 namespace aura {
 
 // This class keeps track of a set of windows. A Window is removed either
-// explicitly by Remove(), or implicitly when the window is destroyed.
+// explicitly by Remove() or Pop(), or implicitly when the window is destroyed.
 class AURA_EXPORT WindowTracker : public WindowObserver {
  public:
   typedef std::set<Window*> Windows;
+  using WindowList = std::vector<Window*>;
 
   WindowTracker();
+  explicit WindowTracker(const WindowList& windows);
   ~WindowTracker() override;
 
+  bool has_windows() const { return !windows_.empty(); }
+
   // Returns the set of windows being observed.
   const std::set<Window*>& windows() const { return windows_; }
 
@@ -36,6 +41,9 @@ class AURA_EXPORT WindowTracker : public WindowObserver {
   // deleted.
   bool Contains(Window* window);
 
+  // Removes and returns the window object from the tracking windows.
+  aura::Window* Pop();
+
   // WindowObserver overrides:
   void OnWindowDestroying(Window* window) override;