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

src/runtime/runtime-debug.cc

Index: src/runtime/runtime-debug.cc
diff --git a/src/runtime/runtime-debug.cc b/src/runtime/runtime-debug.cc
index 5b7df8a5184a10f3d026e1f5e27b1b51ccaac4f6..6de52e2c6741193fd758faba75b89566d9eb5a32 100644
--- a/src/runtime/runtime-debug.cc
+++ b/src/runtime/runtime-debug.cc
@@ -1280,7 +1280,7 @@ RUNTIME_FUNCTION(Runtime_DebugEvaluate) {
   CONVERT_NUMBER_CHECKED(int, inlined_jsframe_index, Int32, args[2]);
   CONVERT_ARG_HANDLE_CHECKED(String, source, 3);
   CONVERT_BOOLEAN_ARG_CHECKED(disable_break, 4);
-  CONVERT_ARG_HANDLE_CHECKED(Object, context_extension, 5);
+  CONVERT_ARG_HANDLE_CHECKED(HeapObject, context_extension, 5);
 
   StackFrame::Id id = DebugFrameHelper::UnwrapFrameId(wrapped_id);
 
@@ -1304,7 +1304,7 @@ RUNTIME_FUNCTION(Runtime_DebugEvaluateGlobal) {
 
   CONVERT_ARG_HANDLE_CHECKED(String, source, 1);
   CONVERT_BOOLEAN_ARG_CHECKED(disable_break, 2);
-  CONVERT_ARG_HANDLE_CHECKED(Object, context_extension, 3);
+  CONVERT_ARG_HANDLE_CHECKED(HeapObject, context_extension, 3);
 
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(