Oilpan: Remove checkMarkingAllowed

Source/platform/heap/MarkingVisitor.h

 // Copyright 2015 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 MarkingVisitor_h
 #define MarkingVisitor_h
 
 #include "platform/heap/MarkingVisitorImpl.h"
 
 namespace blink {
@@ skipping 172 matching lines @@
         if (Mode != ThreadLocalMarking)
             return true;
 
         BasePage* page = pageFromObject(objectPointer);
         ASSERT(!page->orphaned());
         // When doing a thread local GC, the marker checks if
         // the object resides in another thread's heap. If it
         // does, the object should not be marked & traced.
         return page->terminating();
     }
-
-#if ENABLE(ASSERT)
-    virtual void checkMarkingAllowed() override
-    {
-        ASSERT(ThreadState::current()->isInGC());
-    }
-#endif
 };
 
 } // namespace blink
 
 #endif