Add collection of usage signatures on PeerConnections

This generates a number that represent a set of bits that
indicates how a PeerConnection has been used over time.

Bug: chromium:718508
Change-Id: I6df177684c50bc825bc41ea97996574292084d41
Reviewed-on: https://webrtc-review.googlesource.com/79823
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23471}

api/umametrics.h

@@ -45,6 +45,7 @@ enum PeerConnectionEnumCounterType {
   // The next 2 counters log the value of srtp_err_status_t defined in libsrtp.
   kEnumCounterSrtpUnprotectError,
   kEnumCounterSrtcpUnprotectError,
+  kEnumCounterUsagePattern,
   kPeerConnectionEnumCounterMax
 };