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}

p2p/base/testturncustomizer.h

@@ -0,0 +1,57 @@
+/*
+ *  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 P2P_BASE_TESTTURNCUSTOMIZER_H_
+#define P2P_BASE_TESTTURNCUSTOMIZER_H_
+
+#include "api/turncustomizer.h"
+#include "rtc_base/ptr_util.h"
+
+namespace cricket {
+
+class TestTurnCustomizer : public webrtc::TurnCustomizer {
+ public:
+  TestTurnCustomizer() {}
+  virtual ~TestTurnCustomizer() {}
+
+  enum TestTurnAttributeExtensions {
+    // Test only attribute
+    STUN_ATTR_COUNTER                     = 0xFF02   // Number
+  };
+
+  void MaybeModifyOutgoingStunMessage(
+      cricket::PortInterface* port,
+      cricket::StunMessage* message) override {
+    modify_cnt_ ++;
+
+    if (add_counter_) {
+      message->AddAttribute(rtc::MakeUnique<cricket::StunUInt32Attribute>(
+          STUN_ATTR_COUNTER, modify_cnt_));
+    }
+    return;
+  }
+
+  bool AllowChannelData(cricket::PortInterface* port,
+                        const void* data,
+                        size_t size,
+                        bool payload) override {
+    allow_channel_data_cnt_++;
+    return allow_channel_data_;
+  }
+
+  bool add_counter_ = false;
+  bool allow_channel_data_ = true;
+  unsigned int modify_cnt_ = 0;
+  unsigned int allow_channel_data_cnt_ = 0;
+};
+
+}  // namespace cricket
+
+#endif  // P2P_BASE_TESTTURNCUSTOMIZER_H_