Fix DeviceStatusCollectorTest.MaxStoredPeriods.

base/test/scoped_environment_variable.cc

Index: base/test/scoped_environment_variable.cc
diff --git a/base/test/scoped_environment_variable.cc b/base/test/scoped_environment_variable.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a044111e16cbe5d74528beb085ad5e054e655f95
--- /dev/null
+++ b/base/test/scoped_environment_variable.cc
@@ -0,0 +1,27 @@
+// Copyright (c) 2012 The Chromium 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 "base/test/scoped_environment_variable.h"
+
+#include "base/environment.h"
+
+namespace base {
+
+ScopedEnvironmentVariable::ScopedEnvironmentVariable(
+    const std::string& name,
+    const std::string& value)
+    : name_(name),
+      environment_(base::Environment::Create()) {
+  prev_was_set_ = environment_->GetVar(name.c_str(), &prev_value_);
+  environment_->SetVar(name_.c_str(), value);
+}
+
+ScopedEnvironmentVariable::~ScopedEnvironmentVariable() {
+  if (prev_was_set_)
+    environment_->SetVar(name_.c_str(), prev_value_);
+  else
+    environment_->UnSetVar(name_.c_str());
+}
+
+}  // namespace base