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

ui/aura/window_tracker.cc

Index: ui/aura/window_tracker.cc
diff --git a/ui/aura/window_tracker.cc b/ui/aura/window_tracker.cc
index 9653e5fcf9ea57dc2b147e0e2f34b6d041cdef9b..5ff1a0fc429e842a1102296d74657a8249263a63 100644
--- a/ui/aura/window_tracker.cc
+++ b/ui/aura/window_tracker.cc
@@ -11,9 +11,15 @@ 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) {
@@ -35,6 +41,13 @@ 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);