Introduce explicit constant for per Context debug data set by embedder

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 755a87fdba14f78c70e9ec8bd05a6852a65a49ce..a7c86fd0ba3e11adf361896245b602292644ff6e 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -6255,6 +6255,13 @@ class V8_EXPORT Context {
   v8::Isolate* GetIsolate();
 
   /**
+   * The field at kDebugIdIndex is reserved for V8 debugger implementation.
+   * The value is propagated to the scripts compiled in given Context and
+   * can be used for filtering scripts.
+   */
+  enum EmbedderDataFields { kDebugIdIndex = 0 };
+
+  /**
    * Gets the embedder data with the given index, which must have been set by a
    * previous call to SetEmbedderData with the same index. Note that index 0
    * currently has a special meaning for Chrome's debugger.