Refactor and push window properties up to class properties.

ui/base/property_data.h

Index: ui/base/property_data.h
diff --git a/ui/base/property_data.h b/ui/base/property_data.h
new file mode 100644
index 0000000000000000000000000000000000000000..abe13cc746b7cd89cf72367be55179be070c311b
--- /dev/null
+++ b/ui/base/property_data.h
@@ -0,0 +1,22 @@
+// Copyright 2017 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.
+
+#ifndef UI_BASE_PROPERTY_DATA_H_
+#define UI_BASE_PROPERTY_DATA_H_
+
+#include "ui/base/ui_base_export.h"
+
+namespace ui {
+
+// Descendants of ui::PropertyHandler may return a descendant of this class
+// when overriding BeforePropertyChange(). This instance is then passed to
+// AfterPropertyChange() in order to preserve and/or communicate data between
+// those two calls.
+struct UI_BASE_EXPORT PropertyData {
+  virtual ~PropertyData() {}
+};
+
+}
+
+#endif