Unconditionally build VP9 support.

webrtc/modules/video_coding/codecs/vp9/vp9_dummy_impl.cc

-/*
- *  Copyright (c) 2015 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.
- *
- */
-
-// This file contains an implementation of empty webrtc VP9 encoder/decoder
-// factories so it is possible to build webrtc without linking with vp9.
-#include "webrtc/modules/video_coding/codecs/vp9/vp9_impl.h"
-
-namespace webrtc {
-VP9Encoder* VP9Encoder::Create() { return nullptr; }
-VP9Decoder* VP9Decoder::Create() { return nullptr; }
-}