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

src/debug/debug-evaluate.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_DEBUG_DEBUG_EVALUATE_H_
 #define V8_DEBUG_DEBUG_EVALUATE_H_
 
 #include "src/frames.h"
 #include "src/objects.h"
 
 namespace v8 {
 namespace internal {
 
 class DebugEvaluate : public AllStatic {
  public:
   static MaybeHandle<Object> Global(Isolate* isolate, Handle<String> source,
                                     bool disable_break,
-                                    Handle<Object> context_extension);
+                                    Handle<HeapObject> context_extension);
 
   // Evaluate a piece of JavaScript in the context of a stack frame for
   // debugging.  Things that need special attention are:
   // - Parameters and stack-allocated locals need to be materialized.  Altered
   //   values need to be written back to the stack afterwards.
   // - The arguments object needs to materialized.
   static MaybeHandle<Object> Local(Isolate* isolate, StackFrame::Id frame_id,
                                    int inlined_jsframe_index,
                                    Handle<String> source, bool disable_break,
-                                   Handle<Object> context_extension);
+                                   Handle<HeapObject> context_extension);
 
  private:
   // This class builds a context chain for evaluation of expressions
   // in debugger.
   // The scope chain leading up to a breakpoint where evaluation occurs
   // looks like:
   // - [a mix of with, catch and block scopes]
   //    - [function stack + context]
   //      - [outer context]
   // The builder materializes all stack variables into properties of objects;
@@ skipping 42 matching lines @@
     Handle<Context> innermost_context_;
     List<ContextChainElement> context_chain_;
     Isolate* isolate_;
     JavaScriptFrame* frame_;
     int inlined_jsframe_index_;
   };
 
   static MaybeHandle<Object> Evaluate(Isolate* isolate,
                                       Handle<SharedFunctionInfo> outer_info,
                                       Handle<Context> context,
-                                      Handle<Object> context_extension,
+                                      Handle<HeapObject> context_extension,
                                       Handle<Object> receiver,
                                       Handle<String> source);
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_DEBUG_DEBUG_EVALUATE_H_