Add UMA labels to match WebRTC enum

To be submitted when to https://webrtc-review.googlesource.com/c/src/+/113321 is imported in Chrome.

Bug: chromium:914452
Change-Id: Ibea51d0e41687bd775085fa7198cc59d528e8eb7
Reviewed-on: https://chromium-review.googlesource.com/c/1374744
Reviewed-by: Steve Anton <steveanton@chromium.org>
Commit-Queue: Jeroen de Borst <jeroendb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616450}