Address vptr race condition while PeerConnection is destructed.

Auxiliary threads (worker, network) are still active
while PeerConnection is destructed, leading to race condition
in tests such as:
  * RTCStatsIntegrationTest.GetStatsFromCaller
  * RTCStatsIntegrationTest.GetsStatsWhileDestroyingPeerConnection

This CL prevents the conflict to happen by explicitly
closing the PeerConnection.

Bug: webrtc:9847
Change-Id: I40880bb9b193201711031b8c4563c6bbd4983c71
Reviewed-on: https://webrtc-review.googlesource.com/c/104820
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25801}