Wire up UMA for key exchange protocol by media type

Bug: chromium:804275
Change-Id: Id76cce71074ddde14b7e5179d1e536d0658154a1
Reviewed-on: https://chromium-review.googlesource.com/951785
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541761}

content/renderer/media/webrtc/rtc_peer_connection_handler.cc

@@ -844,6 +844,12 @@ class PeerConnectionUMAObserver : public webrtc::UMAObserver {
             static_cast<webrtc::SdpSemanticNegotiated>(counter),
             webrtc::kSdpSemanticNegotiatedMax);
         break;
+      case webrtc::kEnumCounterKeyProtocolMediaType:
+        UMA_HISTOGRAM_ENUMERATION(
+            "WebRTC.PeerConnection.KeyProtocolByMedia",
+            static_cast<webrtc::KeyExchangeProtocolMedia>(counter),
+            webrtc::kEnumCounterKeyProtocolMediaTypeMax);
+        break;
       default:
         // The default clause is expected to be reached when new enum types are
         // added.

tools/metrics/histograms/enums.xml

@@ -34659,6 +34659,15 @@ Called by update_net_trust_anchors.py.-->
   <int value="1" label="SDES"/>
 </enum>
 
+<enum name="PeerConnectionKeyProtocolByMedia">
+  <int value="0" label="DTLS Audio"/>
+  <int value="1" label="DTLS Video"/>
+  <int value="2" label="DTLS Data"/>
+  <int value="3" label="SDES Audio"/>
+  <int value="4" label="SDES Video"/>
+  <int value="5" label="SDES Data"/>
+</enum>
+
 <enum name="PeerConnectionRtcpMux">
   <int value="0" label="Disabled"/>
   <int value="1" label="Enabled"/>

tools/metrics/histograms/histograms.xml

@@ -101294,6 +101294,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.PeerConnection.KeyProtocolByMedia"
+    enum="PeerConnectionKeyProtocolByMedia">
+  <owner>hta@chromium.org</owner>
+  <summary>
+    What key exchange protocol (DTLS or SDES) is used to establish the crypto
+    keys for a PeerConnection's RTP transport, specified per media type.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.PeerConnection.RtcpMux" enum="PeerConnectionRtcpMux">
   <owner>pthatcher@chromium.org</owner>
   <summary>