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

src/contexts-inl.h

 // Copyright 2015 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_CONTEXTS_INL_H_
 #define V8_CONTEXTS_INL_H_
 
 #include "src/contexts.h"
 #include "src/objects-inl.h"
 
@@ skipping 38 matching lines @@
 
 
 Context* Context::previous() {
   Object* result = get(PREVIOUS_INDEX);
   DCHECK(IsBootstrappingOrValidParentContext(result, this));
   return reinterpret_cast<Context*>(result);
 }
 void Context::set_previous(Context* context) { set(PREVIOUS_INDEX, context); }
 
 
-bool Context::has_extension() { return extension() != nullptr; }
-Object* Context::extension() { return get(EXTENSION_INDEX); }
-void Context::set_extension(Object* object) { set(EXTENSION_INDEX, object); }
+bool Context::has_extension() { return !extension()->IsTheHole(); }
+HeapObject* Context::extension() {
+  return HeapObject::cast(get(EXTENSION_INDEX));
+}
+void Context::set_extension(HeapObject* object) {
+  set(EXTENSION_INDEX, object);
+}
 
 
 JSModule* Context::module() { return JSModule::cast(get(EXTENSION_INDEX)); }
 void Context::set_module(JSModule* module) { set(EXTENSION_INDEX, module); }
 
 
 Context* Context::native_context() {
   Object* result = get(NATIVE_CONTEXT_INDEX);
   DCHECK(IsBootstrappingOrNativeContext(this->GetIsolate(), result));
   return reinterpret_cast<Context*>(result);
@@ skipping 67 matching lines @@
     return type::cast(get(index));                        \
   }
 NATIVE_CONTEXT_FIELDS(NATIVE_CONTEXT_FIELD_ACCESSORS)
 #undef NATIVE_CONTEXT_FIELD_ACCESSORS
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CONTEXTS_INL_H_