react-native-webview: Add a libdef.
react-native-webview: Add a libdef.

Generated with `yarn update-libdefs`.  This will allow us to mark
`strict-local` on files that import this module.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile