[Extensions Bindings] Fix getPlatform() method

extensions/renderer/bindings/api_binding_util.h

+// 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.
+
+#ifndef EXTENSIONS_RENDERER_BINDING_API_BINDING_UTIL_H_
+#define EXTENSIONS_RENDERER_BINDING_API_BINDING_UTIL_H_
+
+#include <string>
+
+namespace extensions {
+namespace binding {
+
+// Returns the string version of the current platform, one of "chromeos",
+// "linux", "win", or "mac".
+std::string GetPlatformString();
+
+}  // namespace binding
+}  // namespace extensions
+
+#endif  // EXTENSIONS_RENDERER_BINDING_API_BINDING_UTIL_H_