Introduce phantom weak handles in the API and use them internally for debug info

src/globals.h

 // Copyright 2012 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_GLOBALS_H_
 #define V8_GLOBALS_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 274 matching lines @@
 const Address kZapValue = reinterpret_cast<Address>(0xdeadbeef);
 const Address kHandleZapValue = reinterpret_cast<Address>(0xbaddeaf);
 const Address kGlobalHandleZapValue = reinterpret_cast<Address>(0xbaffedf);
 const Address kFromSpaceZapValue = reinterpret_cast<Address>(0xbeefdaf);
 const uint32_t kSlotsZapValue = 0xbeefdeef;
 const uint32_t kDebugZapValue = 0xbadbaddb;
 const uint32_t kFreeListZapValue = 0xfeed1eaf;
 #endif
 
 const int kCodeZapValue = 0xbadc0de;
+const uint32_t kPhantomReferenceZap = 0xca11bac;
 
 // On Intel architecture, cache line size is 64 bytes.
 // On ARM it may be less (32 bytes), but as far this constant is
 // used for aligning data, it doesn't hurt to align on a greater value.
 #define PROCESSOR_CACHE_LINE_SIZE 64
 
 // Constants relevant to double precision floating point numbers.
 // If looking only at the top 32 bits, the QNaN mask is bits 19 to 30.
 const uint32_t kQuietNaNHighBitsMask = 0xfff << (51 - 32);
 
@@ skipping 500 matching lines @@
 
 inline bool IsConciseMethod(FunctionKind kind) {
   DCHECK(IsValidFunctionKind(kind));
   return kind & FunctionKind::kConciseMethod;
 }
 } }  // namespace v8::internal
 
 namespace i = v8::internal;
 
 #endif  // V8_GLOBALS_H_