Use v8::Context::kDebugIdIndex instead of hardwired constant in gin

gin/public/context_holder.h

Index: gin/public/context_holder.h
diff --git a/gin/public/context_holder.h b/gin/public/context_holder.h
index afbcf23d73933c6daf4756c535f0340ea93cdab3..b055b9680d421e8db36ad6fd0b9e0792c60151b0 100644
--- a/gin/public/context_holder.h
+++ b/gin/public/context_holder.h
@@ -19,7 +19,7 @@ namespace gin {
 // The field at kDebugIdIndex is treated specially by V8 and is reserved for
 // a V8 debugger implementation (not used by gin).
 enum ContextEmbedderDataFields {
-  kDebugIdIndex = 0,
+  kDebugIdIndex = v8::Context::kDebugIdIndex,
   kPerContextDataStartIndex,
 };