Fix a dangling-pointer bug in P2PTransportChannel unit tests.

Messages posted to the main thread are handled by dangling pointers to
destroyed endpoints after the tests end.

Bug: None
Change-Id: I901d63bf11574882d0259cd4b07e628f6a4eebc5
Reviewed-on: https://webrtc-review.googlesource.com/84540
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Qingsi Wang <qingsi@google.com>
Cr-Commit-Position: refs/heads/master@{#23691}
diff --git a/p2p/base/p2ptransportchannel_unittest.cc b/p2p/base/p2ptransportchannel_unittest.cc
index 89018ab..57af9dc 100644
--- a/p2p/base/p2ptransportchannel_unittest.cc
+++ b/p2p/base/p2ptransportchannel_unittest.cc
@@ -394,6 +394,7 @@
     return channel;
   }
   void DestroyChannels() {
+    main_.Clear(this);
     ep1_.cd1_.ch_.reset();
     ep2_.cd1_.ch_.reset();
     ep1_.cd2_.ch_.reset();