[landing] changed webview bridge message to include signature

Summary: passed the wrong parameter, change got lost in reversion

Test Plan: run landing on a simulator or physical device and get a message over the bridge instead of nothing

Reviewers: ashoat, atul, varun, abosh

Reviewed By: ashoat

Subscribers: tomek, abosh, varun, atul, ashoat

Differential Revision: https://phab.comm.dev/D5152