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

BUG=466631

Review URL: https://codereview.chromium.org/1011553003

Cr-Commit-Position: refs/heads/master@{#321534}

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,
 };