CC Animation: Unify ElementId in Blink Compositor Worker.

cc/trees/element_id.h

Index: cc/trees/element_id.h
diff --git a/cc/trees/element_id.h b/cc/trees/element_id.h
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index 0000000000000000000000000000000000000000..137b3830d48aedda49e3c6c8bc6662b4410bc4f3
--- /dev/null
+++ b/cc/trees/element_id.h
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+// Copyright 2016 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 CC_TREES_ELEMENT_ID_H_
+#define CC_TREES_ELEMENT_ID_H_
+
+#include "cc/base/cc_export.h"
+
+namespace cc {
+
+// A CC client-provided id to attach effects like animation to an
+// abstract element (a group of layers or property tree nodes). Must be unique.
+using ElementId = int;
+
+}  // namespace cc
+
+#endif  // CC_TREES_ELEMENT_ID_H_