[turbofan] Make Factory::NewNumber() always return the minus_zero_value.

test/unittests/factory-unittest.cc

+// 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.
+
+#include "src/factory.h"
+#include "src/handles-inl.h"
+#include "test/unittests/test-utils.h"
+
+namespace v8 {
+namespace internal {
+
+typedef TestWithIsolate FactoryTest;
+
+
+TEST_F(FactoryTest, NewNumberWithMinusZero) {
+  Handle<Object> minus_zero_value = factory()->minus_zero_value();
+  EXPECT_TRUE(minus_zero_value.is_identical_to(factory()->NewNumber(-0.0)));
+}
+
+}  // namespace internal
+}  // namespace v8