Add trace method for NoopUnderlyingSource GC mixin.

Source/core/streams/ExclusiveStreamReaderTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/streams/ExclusiveStreamReader.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "bindings/core/v8/ToV8.h"
@@ skipping 36 matching lines @@
     }
 
     String* m_value;
 };
 
 class NoopUnderlyingSource final : public GarbageCollectedFinalized<NoopUnderlyingSource>, public UnderlyingSource {
     USING_GARBAGE_COLLECTED_MIXIN(NoopUnderlyingSource);
 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); }
 };
 
 class PermissiveStrategy final : public StringStream::Strategy {
 public:
     bool shouldApplyBackpressure(size_t, ReadableStream*) override { return false; }
 };
 
 class ExclusiveStreamReaderTest : public ::testing::Test {
@@ skipping 357 matching lines @@
     reader->releaseLock();
     EXPECT_TRUE(onFulfilled.isNull());
 
     isolate()->RunMicrotasks();
     EXPECT_EQ("undefined", onFulfilled);
 }
 
 } // namespace
 
 } // namespace blink