Log DTLS writable changes to RtcEventLog

Bug: webrtc:9972
Change-Id: I8d29dcbc200bd0aad7f4c2600459c4d344c61bbb
Reviewed-on: https://webrtc-review.googlesource.com/c/111453
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Zach Stein <zstein@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25866}
diff --git a/p2p/base/dtlstransport.cc b/p2p/base/dtlstransport.cc
index 51091a3..75e0c1b 100644
--- a/p2p/base/dtlstransport.cc
+++ b/p2p/base/dtlstransport.cc
@@ -16,6 +16,7 @@
 
 #include "absl/memory/memory.h"
 #include "logging/rtc_event_log/events/rtc_event_dtls_transport_state.h"
+#include "logging/rtc_event_log/events/rtc_event_dtls_writable_state.h"
 #include "logging/rtc_event_log/rtc_event_log.h"
 #include "p2p/base/packettransportinternal.h"
 #include "rtc_base/buffer.h"
@@ -751,6 +752,10 @@
   if (writable_ == writable) {
     return;
   }
+  if (event_log_) {
+    event_log_->Log(
+        absl::make_unique<webrtc::RtcEventDtlsWritableState>(writable));
+  }
   RTC_LOG(LS_VERBOSE) << ToString() << ": set_writable to: " << writable;
   writable_ = writable;
   if (writable_) {