[Extensions Bindings] Fix getPlatform() method

extensions/renderer/bindings/api_binding_util.cc

+// Copyright 2017 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 "extensions/renderer/bindings/api_binding_util.h"
+
+#include "base/logging.h"
+#include "build/build_config.h"
+
+namespace extensions {
+namespace binding {
+
+std::string GetPlatformString() {
+#if defined(OS_CHROMEOS)
+  return "chromeos";
+#elif defined(OS_LINUX)
+  return "linux";
+#elif defined(OS_MACOSX)
+  return "mac";
+#elif defined(OS_WIN)
+  return "win";
+#else
+  NOTREACHED();
+  return std::string();
+#endif
+}
+
+}  // namespace binding
+}  // namespace extensions