Relanding "Generate localhost candidate when no STUN/TURN and portallocator has the right flag spefied."

Migrated from https://codereview.webrtc.org/1275703006/ which causes test failures for android. On android, loopback interface was used as local interface to generate candidates. Add a test case to make sure this won't be broken in the future.

Also observed some failures under content_browsertests in chromium.fyi bot but can't repro locally. Might just be temporary test issue.

BUG=webrtc:4517
TBR=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1299333003 .

Cr-Commit-Position: refs/heads/master@{#9746}
13 files changed