Reland of Loosening the coupling between WebRTC and //third_party/protobuf

webrtc/base/protobuf_utils.h

+/*
+ *  Copyright (c) 2017 The WebRTC 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 in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <string>
+
+#ifndef WEBRTC_BASE_PROTOBUF_UTILS_H_
+#define WEBRTC_BASE_PROTOBUF_UTILS_H_
+
+namespace webrtc {
+
+using ProtoString = std::string;
+
+}  // namespace webrtc
+
+#if WEBRTC_ENABLE_PROTOBUF
+
+#include "third_party/protobuf/src/google/protobuf/message_lite.h"
+#include "third_party/protobuf/src/google/protobuf/repeated_field.h"
+
+namespace webrtc {
+
+using google::protobuf::MessageLite;
+using google::protobuf::RepeatedPtrField;
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_ENABLE_PROTOBUF
+
+#endif  // WEBRTC_BASE_PROTOBUF_UTILS_H_