Introduce NetEq::Config::ToString and use it in NetEq's constructor

R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/54559004

Cr-Original-Commit-Position: refs/heads/master@{#9279}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 905495cfaaed396c816f0a62e31e0311ce86e1b0
3 files changed