[heap] Do not invoke GC to make heap iterable.

test/mjsunit/debug-referenced-by.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 69 matching lines @@
 // This adds a references (function context).
 g = closure_eval(a);
 assertEquals(6, mirror.referencedBy().length);
 
 // Dynamically create a variable. This should create a context extension.
 h = closure_eval(null, "var x_");
 assertEquals(6, mirror.referencedBy().length);
 // Adds a reference when set.
 h("x_ = a");
 var x = mirror.referencedBy();
-assertEquals(7, mirror.referencedBy().length);
+assertTrue(mirror.referencedBy().length <= 8);
 // Removes a reference when cleared.
 h("x_ = null");
 assertEquals(6, mirror.referencedBy().length);
 
 // Check circular references.
 x = {}
 mirror = debug.MakeMirror(x);
 assertEquals(1, mirror.referencedBy().length);
 x.x = x;
 assertEquals(2, mirror.referencedBy().length);
 x = null;
 assertEquals(0, mirror.referencedBy().length);
 
 // Check many references.
 y = {}
 mirror = debug.MakeMirror(y);
 refs = [];
 for (var i = 0; i < 200; i++) {
   refs[i] = {'y': y};
 }
 y = null;
 assertEquals(200, mirror.referencedBy().length);