Ensure that Heap.h (not Visitor.h) is #included from Oilpan user code

Source/platform/heap/Visitor.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 11 matching lines @@
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
+#define Visitor_cpp
 #include "platform/heap/Visitor.h"
 
 #include "platform/heap/Handle.h"
 #include "platform/heap/Heap.h"
 
 namespace blink {
 
 // GCInfo indices start from 1 for heap objects, with 0 being treated
 // specially as the index for freelist entries and large heap objects.
 int GCInfoTable::s_gcInfoIndex = 0;
@@ skipping 60 matching lines @@
 #if !defined(COMPONENT_BUILD)
     // On component builds we cannot compare the gcInfos as they are statically
     // defined in each of the components and hence will not match.
     BasePage* page = pageFromObject(payload);
     ASSERT(page->orphaned() || HeapObjectHeader::fromPayload(payload)->gcInfoIndex() == gcInfoIndex);
 #endif
 }
 #endif
 
 }