[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/profiler/heap-snapshot-generator.cc

 // Copyright 2013 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.
 
 #include "src/profiler/heap-snapshot-generator.h"
 
 #include "src/code-stubs.h"
 #include "src/conversions.h"
 #include "src/debug/debug.h"
 #include "src/objects-body-descriptors.h"
@@ skipping 1229 matching lines @@
   if (Context::index < Context::FIRST_WEAK_SLOT || \
       Context::index == Context::MAP_CACHE_INDEX) { \
     SetInternalReference(context, entry, #name, context->get(Context::index), \
         FixedArray::OffsetOfElementAt(Context::index)); \
   } else { \
     SetWeakReference(context, entry, #name, context->get(Context::index), \
         FixedArray::OffsetOfElementAt(Context::index)); \
   }
   EXTRACT_CONTEXT_FIELD(CLOSURE_INDEX, JSFunction, closure);
   EXTRACT_CONTEXT_FIELD(PREVIOUS_INDEX, Context, previous);
-  EXTRACT_CONTEXT_FIELD(EXTENSION_INDEX, Object, extension);
+  EXTRACT_CONTEXT_FIELD(EXTENSION_INDEX, HeapObject, extension);
   EXTRACT_CONTEXT_FIELD(NATIVE_CONTEXT_INDEX, Context, native_context);
   if (context->IsNativeContext()) {
     TagObject(context->normalized_map_cache(), "(context norm. map cache)");
     TagObject(context->embedder_data(), "(context data)");
     NATIVE_CONTEXT_FIELDS(EXTRACT_CONTEXT_FIELD)
     EXTRACT_CONTEXT_FIELD(OPTIMIZED_FUNCTIONS_LIST, unused,
                           optimized_functions_list);
     EXTRACT_CONTEXT_FIELD(OPTIMIZED_CODE_LIST, unused, optimized_code_list);
     EXTRACT_CONTEXT_FIELD(DEOPTIMIZED_CODE_LIST, unused, deoptimized_code_list);
     EXTRACT_CONTEXT_FIELD(NEXT_CONTEXT_LINK, unused, next_context_link);
@@ skipping 1906 matching lines @@
   for (int i = 1; i < sorted_strings.length(); ++i) {
     writer_->AddCharacter(',');
     SerializeString(sorted_strings[i]);
     if (writer_->aborted()) return;
   }
 }
 
 
 }  // namespace internal
 }  // namespace v8