CC Animation: Unify ElementId in Blink Compositor Worker.

cc/trees/mutator_host_client.h

Index: cc/trees/mutator_host_client.h
diff --git a/cc/trees/mutator_host_client.h b/cc/trees/mutator_host_client.h
index 1e1ec5e15012bfc2df7d01aa14770c3ea25a870c..8365c13a43b2bea1d41726eaf947f86cd4c9cc60 100644
--- a/cc/trees/mutator_host_client.h
+++ b/cc/trees/mutator_host_client.h
@@ -5,6 +5,8 @@
 #ifndef CC_TREES_MUTATOR_HOST_CLIENT_H_
 #define CC_TREES_MUTATOR_HOST_CLIENT_H_
 
+#include "cc/trees/element_id.h"
+
 namespace gfx {
 class Transform;
 class ScrollOffset;
@@ -14,8 +16,6 @@ namespace cc {
 
 class FilterOperations;
 
-using ElementId = int;
-
 enum class ElementListType { ACTIVE, PENDING };
 
 class MutatorHostClient {