Add UMA metrics and finch experiment for DTLS1.2 in WebRTC.

content/renderer/media/webrtc/peer_connection_dependency_factory.cc

Index: content/renderer/media/webrtc/peer_connection_dependency_factory.cc
diff --git a/content/renderer/media/webrtc/peer_connection_dependency_factory.cc b/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
index 663f61869c45cc70b43a6b0e2931f2af057effe6..4ad6e2ea6b3cc3506437ce1ae06413f4947dacf4 100644
--- a/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
+++ b/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
@@ -8,6 +8,8 @@
 
 #include "base/command_line.h"
 #include "base/location.h"
+#include "base/metrics/field_trial.h"
+#include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/synchronization/waitable_event.h"
 #include "content/common/media/media_stream_messages.h"
@@ -375,8 +377,13 @@ void PeerConnectionDependencyFactory::InitializeSignalingThread(
   factory_options.disable_sctp_data_channels = false;
   factory_options.disable_encryption =
       cmd_line->HasSwitch(switches::kDisableWebRtcEncryption);
-  if (cmd_line->HasSwitch(switches::kEnableWebRtcDtls12))
+
+  std::string group_name =
+      base::FieldTrialList::FindFullName("WebRTC-PeerConnectionDTLS1.2");
+  if (cmd_line->HasSwitch(switches::kEnableWebRtcDtls12) ||
+      StartsWith(group_name, "Enabled", base::CompareCase::SENSITIVE)) {
     factory_options.ssl_max_version = rtc::SSL_PROTOCOL_DTLS_12;
+  }
   pc_factory_->SetOptions(factory_options);
 
   event->Signal();