Add StereoCodecAdapter classes

This CL is the step 1 for adding alpha channel support over the wire in webrtc.
- Add the footprint for adapter classes that wraps actual codecs.
- This CL does not add a webrtc::VideoFrame container that can carry alpha to 
make the CL shorter for an easier review. Therefore, it exercises a code path
for when we receive no alpha input, just regular I420 frames.
- Unittest sends a video frame for encode/decode through these adapters and 
checks the output PSNR.
- See https://webrtc-review.googlesource.com/c/src/+/7800 for the experimental 
CL that gives an idea about how it will come together.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: I9d3be13647a0a958feceb8d7a9aa93852fc6a1fa
Reviewed-on: https://webrtc-review.googlesource.com/11841
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20490}

api/test/mock_video_decoder_factory.h

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (c) 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_TEST_MOCK_VIDEO_DECODER_FACTORY_H_
+#define API_TEST_MOCK_VIDEO_DECODER_FACTORY_H_
+
+#include <memory>
+#include <vector>
+
+#include "api/video_codecs/sdp_video_format.h"
+#include "api/video_codecs/video_decoder_factory.h"
+#include "test/gmock.h"
+
+namespace webrtc {
+
+class MockVideoDecoderFactory : public webrtc::VideoDecoderFactory {
+ public:
+  MOCK_CONST_METHOD0(GetSupportedFormats,
+                     std::vector<webrtc::SdpVideoFormat>());
+
+  // We need to proxy to a return type that is copyable.
+  std::unique_ptr<webrtc::VideoDecoder> CreateVideoDecoder(
+      const webrtc::SdpVideoFormat& format) {
+    return std::unique_ptr<webrtc::VideoDecoder>(
+        CreateVideoDecoderProxy(format));
+  }
+  MOCK_METHOD1(CreateVideoDecoderProxy,
+               webrtc::VideoDecoder*(const webrtc::SdpVideoFormat&));
+
+  MOCK_METHOD0(Die, void());
+  ~MockVideoDecoderFactory() { Die(); }
+};
+}  // namespace webrtc
+
+#endif  // API_TEST_MOCK_VIDEO_DECODER_FACTORY_H_