[debugger] whitelist some builtins as side-effect free.

test/debugger/test-api.js

 // Copyright 2016 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.
 
 "use strict";
 
 // If true, prints all messages sent and received by inspector.
 const printProtocolMessages = false;
 
 // The active wrapper instance.
@@ skipping 639 matching lines @@
             break;
           }
         }
         break;
       }
       case "undefined": {
         value = undefined;
         isUndefined = true;
         break;
       }
+      case "number": {
+        if (obj.description === "NaN") {
+          value = NaN;
+        }
+        break;
+      }
       case "string":
-      case "number":
       case "boolean": {
         break;
       }
       default: {
         break;
       }
     }
 
     return { value : () => value,
              isUndefined : () => isUndefined,
@@ skipping 213 matching lines @@
     debug.instance = new DebugWrapper();
     debug.instance.enable();
   }
   return debug.instance;
 }});
 
 Object.defineProperty(debug, 'ScopeType', { get: function() {
   const instance = debug.Debug;
   return instance.ScopeType;
 }});