Fix a crasher with "select file" dialogs.

app/win/window_impl.h

Index: app/win/window_impl.h
diff --git a/app/win/window_impl.h b/app/win/window_impl.h
index ad1fc870a2f6c4feb79b86db1b8ebb59ed6064fe..67f3bf76757fd6fd9ab076884bf6241e4348f059 100644
--- a/app/win/window_impl.h
+++ b/app/win/window_impl.h
@@ -70,6 +70,9 @@ class WindowImpl : public MessageMapInterface {
   }
   UINT initial_class_style() const { return class_style_; }
 
+  // Returns true if the specified |hwnd| is a WindowImpl.
+  static bool IsWindowImpl(HWND hwnd);
+
  protected:
   // Handles the WndProc callback for this object.
   virtual LRESULT OnWndProc(UINT message, WPARAM w_param, LPARAM l_param);