Add regression test for dependency to field type tracked weak map.

test/mjsunit/regress/regress-3969.js

+// 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.
+
+// Flags: --allow-natives-syntax
+
+function Inner() {
+  this.property = "OK";
+  this.o2 = 1;
+}
+
+function Outer(inner) {
+  this.inner = inner;
+}
+
+var inner = new Inner();
+var outer = new Outer(inner);
+
+Outer.prototype.boom = function() {
+  return this.inner.property;
+}
+
+assertEquals("OK", outer.boom());
+assertEquals("OK", outer.boom());
+%OptimizeFunctionOnNextCall(Outer.prototype.boom);
+assertEquals("OK", outer.boom());
+
+inner = undefined;
+%SetAllocationTimeout(0 /*interval*/, 2 /*timeout*/);
+// Call something that will do GC while holding a handle to outer's map.
+// The key is that this lets inner's map die while keeping outer's map alive.
+delete outer.inner;
+
+outer = new Outer({field: 1.51, property: "OK"});
+
+assertEquals("OK", outer.boom());