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: Ibf5cc10af84c57288f1eb4c578ca064611a769f1
Reviewed-on: https://webrtc-review.googlesource.com/4781
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20197}

sdk/android/api/org/webrtc/TurnCustomizer.java

@@ -0,0 +1,27 @@
+/*
+ *  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.
+ */
+
+package org.webrtc;
+
+/** Java wrapper for a C++ TurnCustomizer. */
+public class TurnCustomizer {
+  final long nativeTurnCustomizer;
+
+  public TurnCustomizer(long nativeTurnCustomizer) {
+    this.nativeTurnCustomizer = nativeTurnCustomizer;
+  }
+
+  public void dispose() {
+    nativeFreeTurnCustomizer(nativeTurnCustomizer);
+  }
+
+  private static native void nativeFreeTurnCustomizer(
+      long nativeTurnCustomizer);
+}