Add ToString() methods to classes with << operators, preparing for deprecations.

Bug: webrtc:8982
Change-Id: I9b8792a229539dd9848f4d9936fe343f4bf9ad49
Reviewed-on: https://webrtc-review.googlesource.com/63200
Commit-Queue: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22705}

rtc_base/strings/audio_format_to_string.h

@@ -0,0 +1,24 @@
+/*
+ *  Copyright 2018 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.
+ */
+
+#ifndef RTC_BASE_STRINGS_AUDIO_FORMAT_TO_STRING_H_
+#define RTC_BASE_STRINGS_AUDIO_FORMAT_TO_STRING_H_
+
+#include <string>
+
+#include "api/audio_codecs/audio_format.h"
+
+namespace rtc {
+  std::string ToString(const webrtc::SdpAudioFormat& saf);
+  std::string ToString(const webrtc::AudioCodecInfo& saf);
+  std::string ToString(const webrtc::AudioCodecSpec& acs);
+}  // namespace rtc
+
+#endif  // RTC_BASE_STRINGS_AUDIO_FORMAT_TO_STRING_H_