Fix libjingle to provide a field_trial implementation.
Fix libjingle to provide a field_trial implementation.

This completes https://webrtc-codereview.appspot.com/14489004/ by updating libjingle rules.

BUG=crbug/367114
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20479004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 581e2172af690941d8630895380ce67dab53b31d

sippet/talk,sippet/talk,sippet/talk,sippet/talk,sippet/talk