Ensuring that UDP TURN servers are always used as STUN servers.

This was already working in most cases, but not for some corner cases:
* If the PORTALLOCATOR_ENABLE_SHARED_SOCKET flag is not set
* If both a STUN server and TURN server are configured

I added unit tests for these cases, and centralized the code that gets
STUN server addresses in order to fix these and any related issues.

BUG=webrtc:4215

Review URL: https://codereview.webrtc.org/1215713003

Cr-Commit-Position: refs/heads/master@{#9596}
12 files changed