Add use counters for EnableDtlsSrtp constraint

These will be turned into deprecation warnings when we
start deprecating SDES crypto and the constraint.

Bug: 804275
Change-Id: I0c5763406b9f4bd32890708e34a132ad6553a929
Reviewed-on: https://chromium-review.googlesource.com/941955
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540265}
3 files changed