[Extensions] Pull ExtensionId into its own file

extensions/common/extension_id.h

+// Copyright 2016 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_COMMON_EXTENSION_ID_H_
+#define EXTENSIONS_COMMON_EXTENSION_ID_H_
+
+#include <string>
+
+namespace extensions {
+
+// If valid, uniquely identifies an Extension using 32 characters from the
+// alphabet 'a'-'p'.
+using ExtensionId = std::string;
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_COMMON_EXTENSION_ID_H_