Add comment to use counter def, so it doesn't constantly get out of sync

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 081b6c3810793b92dab625578727e46271689588..624bf5a385e9e42826592c4164a3ea8bfa11e1dc 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5469,6 +5469,9 @@ class V8_EXPORT Isolate {
     kHtmlComment = 21,
     kSloppyModeBlockScopedFunctionRedefinition = 22,
     kForInInitializer = 23,
+
+    // If you add new values here, you'll also need to update V8Initializer.cpp
+    // in Chromium.
     kUseCounterFeatureCount  // This enum value must be last.
   };