Add missing USING_GARBAGE_COLLECTED_MIXIN to RTCQuicStream.

This is required for correct heap tracing, since RTCQuicStream derives
ContextClient, which is a GC mixin.

Bug: 869498
Change-Id: I124c3a23e4a6e670fc3af195fa43498c3f9a857e
Reviewed-on: https://chromium-review.googlesource.com/c/1340790Reviewed-by: Steve Anton <steveanton@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608920}

third_party/blink/renderer/modules/peerconnection/rtc_quic_stream.h

@@ -28,6 +28,7 @@ class MODULES_EXPORT RTCQuicStream final : public EventTargetWithInlineData,
                                            public ContextClient,
                                            public QuicStreamProxy::Delegate {
   DEFINE_WRAPPERTYPEINFO();
+  USING_GARBAGE_COLLECTED_MIXIN(RTCQuicStream);
 
  public:
   // TODO(steveanton): These maybe should be adjustable.