Oilpan: fix build after r192035.

Source/core/html/imports/HTMLImportTreeRoot.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/html/imports/HTMLImportTreeRoot.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/StyleEngine.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 75 matching lines @@
         if (equalIgnoringFragmentIdentifier(candidate->url(), url))
             return candidate;
     }
 
     return nullptr;
 }
 
 void HTMLImportTreeRoot::recalcTimerFired(Timer<HTMLImportTreeRoot>*)
 {
     ASSERT(m_document);
-    RefPtrWillBeRawPtr<Document> protect(m_document);
+    RefPtrWillBeRawPtr<Document> protect(m_document.get());
 
     do {
         m_recalcTimer.stop();
         HTMLImport::recalcTreeState(this);
     } while (m_recalcTimer.isActive());
 }
 
 DEFINE_TRACE(HTMLImportTreeRoot)
 {
     visitor->trace(m_document);
     visitor->trace(m_imports);
     HTMLImport::trace(visitor);
 }
 
 }