Add an ICE nomination attribute. If a connection switched on the controlling side, increase the nomination value set in the attribute.
The controlled side will also be ready for re-nomination option; it will switch if a nomination comes with a higher nomination value even though it may be at a lower priority.
Plus, don't nominate or re-nominate if the nomination value at the current connection has been acknowledged.
BUG=
R=deadbeef@webrtc.org, pthatcher@webrtc.org
Review URL: https://codereview.webrtc.org/2163403002 .
Cr-Commit-Position: refs/heads/master@{#13631}
8cd8f81748