Expose WebRTC counter for key protocol to UMA

This allows to observe how many WebRTC users use the deprecated
SDES key agreement style and how many use the DTLS style.

Bug: 804275
Change-Id: I0220328a0897e67fde387288ad6ec1475dca3de3
Reviewed-on: https://chromium-review.googlesource.com/891224
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532847}
3 files changed