Add trace method for NoopUnderlyingSource GC mixin.

Source/core/streams/ExclusiveStreamReaderTest.cpp

Index: Source/core/streams/ExclusiveStreamReaderTest.cpp
diff --git a/Source/core/streams/ExclusiveStreamReaderTest.cpp b/Source/core/streams/ExclusiveStreamReaderTest.cpp
index f57925b5270fe7d42fc355b5af226e1294e1b868..26de2dfd2a6c1aa9869cfa7d5ae61c9a1eda3efe 100644
--- a/Source/core/streams/ExclusiveStreamReaderTest.cpp
+++ b/Source/core/streams/ExclusiveStreamReaderTest.cpp
@@ -54,6 +54,8 @@ class NoopUnderlyingSource final : public GarbageCollectedFinalized<NoopUnderlyi
 public:
     ~NoopUnderlyingSource() override { }
 
+    void trace(Visitor* visitor) override { UnderlyingSource::trace(visitor); }
+
     void pullSource() override { }
     ScriptPromise cancelSource(ScriptState* scriptState, ScriptValue reason) { return ScriptPromise::cast(scriptState, reason); }
 };