[wrapper-tracing] Simplify tracing by forbidding tracing from within mixins

third_party/WebKit/Source/platform/heap/WrapperVisitor.h

 // Copyright 2016 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 WrapperVisitor_h
 #define WrapperVisitor_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 
@@ skipping 136 matching lines @@
   virtual void markWrapper(const v8::PersistentBase<v8::Value>*) const = 0;
 
   virtual void dispatchTraceWrappers(const TraceWrapperBase*) const = 0;
 #define DECLARE_DISPATCH_TRACE_WRAPPERS(ClassName) \
   virtual void dispatchTraceWrappers(const ClassName*) const = 0;
 
   WRAPPER_VISITOR_SPECIAL_CLASSES(DECLARE_DISPATCH_TRACE_WRAPPERS);
 
 #undef DECLARE_DISPATCH_TRACE_WRAPPERS
 
-  void dispatchTraceWrappers(const void*) const {
-    // This call should never be reached as we access all tracing through
-    // TraceTraits, which will check that we can dispatch at compile time.
-    // This handler is merely here to make adjustAndMarkWrapper compile
-    // for GarbageCollectedMixin objects.
-    NOTREACHED();
-  }
-
   virtual bool markWrapperHeader(HeapObjectHeader*) const = 0;
 
   virtual void markWrappersInAllWorlds(const ScriptWrappable*) const = 0;
 
   void markWrappersInAllWorlds(const void*) const {
     // Empty handler used for WRAPPER_VISITOR_SPECIAL_CLASSES. These types
     // don't require marking wrappers in all worlds, so just nop on those.
   }
 
   virtual void pushToMarkingDeque(
       void (*traceWrappersCallback)(const WrapperVisitor*, const void*),
       HeapObjectHeader* (*heapObjectHeaderCallback)(const void*),
       const void*) const = 0;
 };
 
 #define SPECIALIZE_WRAPPER_TRACING_MARK_TRAIT(ClassName) \
   template <>                                            \
   class CanTraceWrappers<ClassName, false> {             \
    public:                                               \
     static const bool value = true;                      \
   };
 
 WRAPPER_VISITOR_SPECIAL_CLASSES(SPECIALIZE_WRAPPER_TRACING_MARK_TRAIT)
 
 #undef SPECIALIZE_WRAPPER_TRACING_MARK_TRAIT
 
 }  // namespace blink
 
 #endif  // WrapperVisitor_h