Reland "Add transaction id to CandidatePairEvents."

This is a reland of c308bdfa451ee2ceac7096b6777fcbf756f4091a

Original change's description:
> Add transaction id to CandidatePairEvents.
>
> The transaction id is a randomly generated number used to link stun
> requests and responses (https://tools.ietf.org/html/rfc5389#section-6).
> Logging this will help us debug ICE network issues.
>
> Bug: webrtc:9972
> Change-Id: I93167cb119aad99156e8727b6e4eeeff5198f924
> Reviewed-on: https://webrtc-review.googlesource.com/c/109720
> Commit-Queue: Zach Stein <zstein@webrtc.org>
> Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
> Reviewed-by: Elad Alon <eladalon@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#25848}

TBR=terelius@webrtc.org,qingsi@webrtc.org

Bug: webrtc:9972
Change-Id: I32b55befddfcb8dc98babd0b64e756eaeb9fab09
Reviewed-on: https://webrtc-review.googlesource.com/c/112661
Reviewed-by: Zach Stein <zstein@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25862}