Port::GetStunMessage: Write to scoped_ptr instead of raw pointer

This is a good idea in general, because it makes ownership clearer,
but will also be very convenient when scoped_ptr is gone, since
unique_ptr doesn't have an .accept() method.

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1800753003

Cr-Original-Commit-Position: refs/heads/master@{#12002}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6baec0351aab267fb4f07b95778ae2e5f0d58f05
3 files changed