The iOS JavaScript Messaging framework (crbug.com/851636) currently relies on the native application to notify the JavaScript on each navigation because the pageshow/pagehide events are not reliable.

This is fixed in iOS 12, so once Chrome is iOS 12+, __gCrWeb.frameMessaging.initializeExistingFrames. should be removed and the JavaScript should notify the native app of the frame's existence on the pageshow event.