Change PeerConnection test helpers to take unique_ptr

This changes DoSet(Local|Remote)Description helper function in
the PeerConnection unit tests to take a unique_ptr to the new
session rather than a bare pointer (of which it took ownership).

Bug: None
Change-Id: I75ef0992f09676455423980972634e3e6a700b85
Reviewed-on: https://chromium-review.googlesource.com/662365
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#19807}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: db45ca80d13f489f390707865accd47b07f70e4c