CC Animation: Move ElementId from animation to trees.

cc/trees/element_id.h

Index: cc/trees/element_id.h
diff --git a/cc/animation/element_id.h b/cc/trees/element_id.h
similarity index 95%
rename from cc/animation/element_id.h
rename to cc/trees/element_id.h
index bca7d6c33e368af2498d270db5fc9a8509359d0c..4c0b652cc889d8080e7e0797e716aed3e8205999 100644
--- a/cc/animation/element_id.h
+++ b/cc/trees/element_id.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef CC_ANIMATION_ELEMENT_ID_H_
-#define CC_ANIMATION_ELEMENT_ID_H_
+#ifndef CC_TREES_ELEMENT_ID_H_
+#define CC_TREES_ELEMENT_ID_H_
 
 #include <stddef.h>
 
@@ -67,4 +67,4 @@ CC_EXPORT std::ostream& operator<<(std::ostream& out, const ElementId& id);
 
 }  // namespace cc
 
-#endif  // CC_ANIMATION_ELEMENT_ID_H_
+#endif  // CC_TREES_ELEMENT_ID_H_