Add a GN target for unit tests, get them working again and added a test.

BUG=webrtc:3417

Review-Url: https://codereview.webrtc.org/2050153003
Cr-Original-Commit-Position: refs/heads/master@{#14959}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bcc5d87f0932e96efdd90dc7fcc50ebab301ce62
3 files changed