Promises are not supported on 4.4: http://caniuse.com/#search=promise
you have to implement a polifill for this.
Have a look at https://github.com/stefanpenner/es6-promise or similar.

Also, just in case you haven't used it before you may want to look into Crosswalk as an alternative WebView. It will provide numerous features for Android 4.4 (at the cost of a larger APK).

look at this:

https://stackoverflow.com/a/48203092/6200607