Add const to BindStateBase to align to RefCounted constness

Review-Url: https://codereview.chromium.org/2344143002
Cr-Commit-Position: refs/heads/master@{#420005}


CrOS-Libchrome-Original-Commit: 30e0c31970dd7d31877f455d8cdb65c8e35f880f
diff --git a/base/bind_internal.h b/base/bind_internal.h
index 5845a4d..17f78a3 100644
--- a/base/bind_internal.h
+++ b/base/bind_internal.h
@@ -470,8 +470,8 @@
 
   ~BindState() {}
 
-  static void Destroy(BindStateBase* self) {
-    delete static_cast<BindState*>(self);
+  static void Destroy(const BindStateBase* self) {
+    delete static_cast<const BindState*>(self);
   }
 };