mojo: Fix crash in example WM when closing window.

Avoid a crash caused by dereferencing an iterator after
erasing it.

BUG=415816

Review URL: https://codereview.chromium.org/580353002

Cr-Commit-Position: refs/heads/master@{#295718}

mojo/examples/window_manager/window_manager.cc

@@ -297,8 +297,9 @@ class WindowManager
   void CloseWindow(Id view_id) {
     WindowVector::iterator iter = GetWindowByViewId(view_id);
     DCHECK(iter != windows_.end());
+    Window* window = *iter;
     windows_.erase(iter);
-    (*iter)->view()->Destroy();
+    window->view()->Destroy();
   }
 
   void ShowKeyboard(Id view_id, const gfx::Rect& bounds) {