[heap] Clear recorded slots for inobject properties when migrating fast object to slow mode.

test/mjsunit/regress/regress-666046.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.
+
+// Flags: --allow-natives-syntax --expose-gc
+
+function P() {
+  this.a0 = {};
+  this.a1 = {};
+  this.a2 = {};
+  this.a3 = {};
+  this.a4 = {};
+}
+
+function A() {
+}
+
+var proto = new P();
+A.prototype = proto;
+
+function foo(o) {
+  return o.a0;
+}
+
+// Ensure |proto| is in old space.
+gc();
+gc();
+gc();
+
+// Ensure |proto| is marked as "should be fast".
+var o = new A();
+foo(o);
+foo(o);
+foo(o);
+assertTrue(%HasFastProperties(proto));
+
+// Contruct a double value that looks like a tagged pointer.
+var buffer = new ArrayBuffer(8);
+var int32view = new Int32Array(buffer);
+var float64view = new Float64Array(buffer);
+int32view[0] = int32view[1] = 0x40000001;
+var boom = float64view[0];
+
+
+// Write new space object.
+proto.a4 = {a: 0};
+// Immediately delete the field.
+delete proto.a4;
+
+// |proto| must sill be fast.
+assertTrue(%HasFastProperties(proto));
+
+// Add a double field instead of deleted a4 that looks like a tagged pointer.
+proto.boom = boom;
+
+// Boom!
+gc();