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

Source/web/WebSocketChannelClientProxy.h

 // 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.
 
 #ifndef WebSocketChannelClientProxy_h
 #define WebSocketChannelClientProxy_h
 
 #include "modules/websockets/WebSocketChannelClient.h"
 #include "platform/heap/Handle.h"
 #include "web/WebSocketImpl.h"
@@ skipping 41 matching lines @@
     {
         m_impl->didStartClosingHandshake();
     }
     virtual void didClose(ClosingHandshakeCompletionStatus status, unsigned short code, const String& reason) override
     {
         WebSocketImpl* impl = m_impl;
         m_impl = nullptr;
         impl->didClose(status, code, reason);
     }
 
+    virtual void trace(Visitor* visitor) override
+    {
+        WebSocketChannelClient::trace(visitor);
+    }
+
 private:
     explicit WebSocketChannelClientProxy(WebSocketImpl* impl)
         : m_impl(impl)
     {
     }
 
     WebSocketImpl* m_impl;
 };
 
 } // namespace blink
 
 #endif // WebSocketChannelClientProxy_h