Add --single-threaded flag that disables background threads.

test/mjsunit/single-threaded.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 --harmony-proxies
+// Flags: --allow-natives-syntax --single-threaded --expose-gc
 
-function bar() {}
-bar({ a: Proxy.create({}) });
-function foo(x) { x.a.b == ""; }
-var x = {a: {b: "" }};
-foo(x);
-foo(x);
+function foo() {
+  for (var i = 0; i < 100; i++) new Array(100);
+}
+
+foo();
+foo();
 %OptimizeFunctionOnNextCall(foo);
-foo(x);
+foo();
+
+gc();
+gc();
+gc();