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

Source/web/WebRemoteFrameImpl.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 "web/WebRemoteFrameImpl.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/RemoteFrame.h"
 #include "core/frame/Settings.h"
@@ skipping 73 matching lines @@
 
 SandboxFlags PlaceholderFrameOwner::sandboxFlags() const
 {
     return 0;
 }
 
 void PlaceholderFrameOwner::dispatchLoad()
 {
 }
 
+void PlaceholderFrameOwner::trace(Visitor* visitor)
+{
+    FrameOwner::trace(visitor);
+}
+
 WebRemoteFrame* WebRemoteFrame::create(WebRemoteFrameClient* client)
 {
     WebRemoteFrameImpl* frame = new WebRemoteFrameImpl(client);
 #if ENABLE(OILPAN)
     return frame;
 #else
     return adoptRef(frame).leakRef();
 #endif
 }
 
@@ skipping 754 matching lines @@
 {
     frame()->setIsLoading(false);
     if (parent() && parent()->isWebLocalFrame()) {
         WebLocalFrameImpl* parentFrame =
             toWebLocalFrameImpl(parent()->toWebLocalFrame());
         parentFrame->frame()->loader().checkCompleted();
     }
 }
 
 } // namespace blink