Iterate more carefully over DTLS transports at close

Ensure that even if the set of DTLS transports is modified during
callbacks called from close, the process will be well-defined.

Bug: chromium:1167357
Change-Id: I712280e7382a647027912178156127831b437f75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639893Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845122}

third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc

@@ -3525,8 +3525,14 @@ void RTCPeerConnection::CloseInternal() {
   if (sctp_transport_) {
     sctp_transport_->Close();
   }
-  for (auto& dtls_transport_iter : dtls_transports_by_native_transport_) {
-    dtls_transport_iter.value->Close();
+  // Since Close() can trigger JS-level callbacks, iterate over a copy
+  // of the transports list.
+  auto dtls_transports_copy = dtls_transports_by_native_transport_;
+  for (auto& dtls_transport_iter : dtls_transports_copy) {
+    // Since "value" is a WeakPtr, check if it's still valid.
+    if (dtls_transport_iter.value) {
+      dtls_transport_iter.value->Close();
+    }
   }
 
   feature_handle_for_scheduler_.reset();