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

test/mjsunit/single-threaded.js

Index: test/mjsunit/single-threaded.js
diff --git a/test/mjsunit/regress/regress-crbug-448730.js b/test/mjsunit/single-threaded.js
similarity index 53%
copy from test/mjsunit/regress/regress-crbug-448730.js
copy to test/mjsunit/single-threaded.js
index 31d276aa839cdf649b2da1ad54138cc4f9418b81..4602e9c450d4bcd5a06ec0d25c2727aa721e9e27 100644
--- a/test/mjsunit/regress/regress-crbug-448730.js
+++ b/test/mjsunit/single-threaded.js
@@ -2,13 +2,17 @@
 // 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
-
-function bar() {}
-bar({ a: Proxy.create({}) });
-function foo(x) { x.a.b == ""; }
-var x = {a: {b: "" }};
-foo(x);
-foo(x);
+// Flags: --allow-natives-syntax --single-threaded --expose-gc
+
+function foo() {
+  for (var i = 0; i < 100; i++) new Array(100);
+}
+
+foo();
+foo();
 %OptimizeFunctionOnNextCall(foo);
-foo(x);
+foo();
+
+gc();
+gc();
+gc();