Add field trials for sending ping on network switches

This patch introduces 2 new field trials that make p2p_transport_channel
to send ping on network switches. The purpose of this is to reduce the
time that the peers disagre on which connection to use.

- send_ping_on_switch_ice_controlling
Send a ping from the ICE_CONTROLLING side when switching connection.
- send_ping_on_nomination_ice_controlled
Send a ping from the ICE_CONTROLLED side when a connection has been
nominated by remote side.

The extra traffic by these PINGS are considered harmless since
network switches does not happen that often.

Bug: webrtc:10273
Change-Id: Id7abe268c79ceb2404c0543849d5666466e58d0c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169550
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30668}