Oilpan: add missing leftmost trace()s for GC mixins.

Source/core/streams/ReadableStreamTest.cpp

 // Copyright 2014 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/ReadableStream.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "bindings/core/v8/ScriptState.h"
@@ skipping 42 matching lines @@
         return value;
     }
 
     String* m_value;
 };
 
 class MockUnderlyingSource : public GarbageCollectedFinalized<MockUnderlyingSource>, public UnderlyingSource {
     USING_GARBAGE_COLLECTED_MIXIN(MockUnderlyingSource);
 public:
     ~MockUnderlyingSource() override { }
+    virtual void trace(Visitor* visitor) override
+    {
+        UnderlyingSource::trace(visitor);
+    }
 
     MOCK_METHOD0(pullSource, void());
     MOCK_METHOD2(cancelSource, ScriptPromise(ScriptState*, ScriptValue));
 };
 
 class PermissiveStrategy : public StringStream::Strategy {
 public:
     bool shouldApplyBackpressure(size_t, ReadableStream*) override { return false; }
 };
 
@@ skipping 695 matching lines @@
     stream->enqueue("bar");
     checkpoint.Call(3);
     EXPECT_TRUE(stream->read(scriptState(), m_exceptionState).toString(chunk));
     EXPECT_EQ("foo", chunk);
     checkpoint.Call(4);
 
     stream->error(DOMException::create(AbortError, "done"));
 }
 
 } // namespace blink