compositor-worker: plumb element id and mutable properties

third_party/WebKit/public/platform/WebMutableProperties.h

+// Copyright 2015 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 WebMutableProperties_h
+#define WebMutableProperties_h
+
+namespace blink {
+
+enum WebMutableProperty {
+    WebMutablePropertyNone = 0,
+    WebMutablePropertyOpacity = 1 << 0,
+    WebMutablePropertyScrollLeft = 1 << 1,
+    WebMutablePropertyScrollTop = 1 << 2,
+    WebMutablePropertyTransform = 1 << 3,
+};
+
+} // namespace blink
+
+#endif // WebMutableProperties_h