Introduce --zap-cpp-pointers (off by default)

src/execution.h

 // Copyright 2014 the V8 project 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 V8_EXECUTION_H_
 #define V8_EXECUTION_H_
 
 #include "src/allocation.h"
 #include "src/base/atomicops.h"
 #include "src/globals.h"
@@ skipping 117 matching lines @@
   }
   Address address_of_real_jslimit() {
     return reinterpret_cast<Address>(&thread_local_.real_jslimit_);
   }
 
   // If the stack guard is triggered, but it is not an actual
   // stack overflow, then handle the interruption accordingly.
   Object* HandleInterrupts();
   void HandleGCInterrupt();
 
+#if DEBUG
+  uintptr_t stack_base_position() { return thread_local_.stack_base_position_; }
+#endif
+
  private:
   StackGuard();
 
   bool CheckInterrupt(InterruptFlag flag);
   void RequestInterrupt(InterruptFlag flag);
   void ClearInterrupt(InterruptFlag flag);
   bool CheckAndClearInterrupt(InterruptFlag flag);
 
   // You should hold the ExecutionAccess lock when calling this method.
   bool has_pending_interrupts(const ExecutionAccess& lock) {
@@ skipping 59 matching lines @@
     uintptr_t climit() {
       return bit_cast<uintptr_t>(base::NoBarrier_Load(&climit_));
     }
     void set_climit(uintptr_t limit) {
       return base::NoBarrier_Store(&climit_,
                                    static_cast<base::AtomicWord>(limit));
     }
 
     PostponeInterruptsScope* postpone_interrupts_;
     int interrupt_flags_;
+#if DEBUG
+    uintptr_t stack_base_position_;
+#endif
   };
 
   // TODO(isolates): Technically this could be calculated directly from a
   //                 pointer to StackGuard.
   Isolate* isolate_;
   ThreadLocal thread_local_;
 
   friend class Isolate;
   friend class StackLimitCheck;
   friend class PostponeInterruptsScope;
 
   DISALLOW_COPY_AND_ASSIGN(StackGuard);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_EXECUTION_H_