TurnCustomizer - an interface for modifying stun messages sent by TurnPort

This patch adds an interface that allows modification of stun messages
sent by TurnPort. A user can inject a TurnCustomizer on the RTCConfig
and the TurnCustomizer will be invoked by TurnPort before sending
message. This allows user to e.g add custom attributes as described
in rtf5389.

BUG=webrtc:8313

Change-Id: I6f4333e9f8ff7fd20f32677be19285f15e1180b6
Reviewed-on: https://webrtc-review.googlesource.com/7618
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20233}

api/turncustomizer.h

@@ -0,0 +1,46 @@
+/*
+ *  Copyright 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.
+ */
+
+#ifndef API_TURNCUSTOMIZER_H_
+#define API_TURNCUSTOMIZER_H_
+
+#include <stdlib.h>
+
+namespace cricket {
+class PortInterface;
+class StunMessage;
+}  // namespace cricket
+
+
+namespace webrtc {
+
+class TurnCustomizer {
+ public:
+  // This is called before a TURN message is sent.
+  // This could be used to add implementation specific attributes to a request.
+  virtual void MaybeModifyOutgoingStunMessage(
+      cricket::PortInterface* port,
+      cricket::StunMessage* message) = 0;
+
+  // TURN can send data using channel data messages or Send indication.
+  // This method should return false if |data| should be sent using
+  // a Send indication instead of a ChannelData message, even if a
+  // channel is bound.
+  virtual bool AllowChannelData(cricket::PortInterface* port,
+                                const void* data,
+                                size_t size,
+                                bool payload) = 0;
+
+  virtual ~TurnCustomizer() {}
+};
+
+}  // namespace webrtc
+
+#endif  // API_TURNCUSTOMIZER_H_