webview js: Add fixme for a Flow issue in new visible-messages code.
webview js: Add fixme for a Flow issue in new visible-messages code.

Apparently neglected to check the final version with Flow -- oops!
Fix that.

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