This API currently needs to be exposed because the frameId is generated at Document Start, but the frame cannot message the native application about it's existance until Document End time.

Once windowID is replaced with frameID, frame registration code should either:
- All be moved to Document End time (f.e. in setup_frame.js)
  or
- Move registration to run all at Document Start (frame_messaging.js), but queue the resulting 'FrameBecameAvailable' message until Document End time.