Adding basic implementation of AudioNetworkAdaptor.

webrtc/modules/audio_coding/audio_network_adaptor/controller.cc

+/*
+ *  Copyright (c) 2016 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 "webrtc/modules/audio_coding/audio_network_adaptor/controller.h"
+
+namespace webrtc {
+
+Controller::NetworkMetrics::NetworkMetrics() = default;
+
+Controller::NetworkMetrics::~NetworkMetrics() = default;
+
+Controller::Constraints::Constraints() = default;
+
+Controller::Constraints::~Constraints() = default;
+
+Controller::Constraints::FrameLengthRange::FrameLengthRange(
+    int min_frame_length_ms,
+    int max_frame_length_ms)
+    : min_frame_length_ms(min_frame_length_ms),
+      max_frame_length_ms(max_frame_length_ms) {}
+
+Controller::Constraints::FrameLengthRange::~FrameLengthRange() = default;
+
+void Controller::SetConstraints(const Constraints& constraints) {}

[kwiberg-webrtc, 2016/09/09 11:56:15]

Do you expect typical controllers to not override this method? Unless overriding
it is rare, it's simpler to just set it to = 0 and require all subclasses to
override it---that way, one doesn't have to look in so many places when trying
to see what a Controller subclass does.

In other words, providing a default implementation carries a cost in increased
complexity, so it should be correspondingly useful.

[minyue-webrtc, 2016/09/12 11:36:11]

Most controllers will not override this method, like AudioEncoder::SetDtx() :)
Do you think the cost of default implementation is acceptable in this case? 

[kwiberg-webrtc, 2016/09/12 13:33:50]

Yes, probably. Thanks for checking.

+
+}  // namespace webrtc