make Connection::port() protected

This patch makes Connection::port() protected
and add explicit methods for the use cases instead
- network()     - port()->Network()
- generation()  - port()->generation()

This is done to easier mock a Connection.

BUG=webrtc:10647

Change-Id: I5b35477ed9f81d57cd871072874262d0a8af2d4c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160784
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29929}