CC Animation: Use ElementId to attach CC animation players.

cc/test/test_element_id.cc

Index: cc/test/test_element_id.cc
diff --git a/cc/test/test_element_id.cc b/cc/test/test_element_id.cc
new file mode 100644
index 0000000000000000000000000000000000000000..588326d1cca1de982cf2c35022e259e923137888
--- /dev/null
+++ b/cc/test/test_element_id.cc
@@ -0,0 +1,15 @@
+// Copyright (c) 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.
+
+#include "cc/test/test_element_id.h"
+
+namespace cc {
+
+ElementId g_next_compositor_element_id = 1;
+
+ElementId NextTestElementId() {
+  return g_next_compositor_element_id++;
+}
+
+}  // namespace cc