Mark ~DirectTransport with "override."

The destructor was not explicitly marked as "virtual" nor as "override".

BUG=None

Review-Url: https://codereview.webrtc.org/2986363002
Cr-Commit-Position: refs/heads/master@{#19236}

webrtc/test/direct_transport.h

@@ -46,7 +46,7 @@ class DirectTransport : public Transport {
             send_call,
             std::unique_ptr<Demuxer>(new ForceDemuxer(MediaType::VIDEO))) {}
 
-  ~DirectTransport();
+  ~DirectTransport() override;
 
   void SetConfig(const FakeNetworkPipe::Config& config);