Fixing memory leak in FakeTransportController.

Introduced by: https://codereview.webrtc.org/2641633002/
Only occurs with test code.

BUG=webrtc:6972
TBR=pthatcher@webrtc.org

Review-Url: https://codereview.webrtc.org/2648093002
Cr-Original-Commit-Position: refs/heads/master@{#16200}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3e4faae0edeb2b30e899712f92217d2aa5cd6ff2