refactor: keep single source of truth (#8334)

src/core/instance/state.js

@@ -86,7 +86,7 @@ function initProps (vm: Component, propsOptions: Object) {
         )
       }
       defineReactive(props, key, value, () => {
-        if (vm.$parent && !isUpdatingChildComponent) {
+        if (!isRoot && !isUpdatingChildComponent) {
           warn(
             `Avoid mutating a prop directly since the value will be ` +
             `overwritten whenever the parent component re-renders. ` +