Move video_encoder.h and video_decoder.h to /api and create GN targets for them

webrtc/modules/video_coding/video_codec_initializer_unittest.cc

 /*
  *  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.
  */
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_video/include/video_bitrate_allocator.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
 #include "webrtc/modules/video_coding/include/video_codec_initializer.h"
 #include "webrtc/test/gtest.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
 namespace {
 static const char* kVp8PayloadName = "VP8";
 static const int kVp8PayloadType = 100;
 static const int kDefaultWidth = 1280;
 static const int kDefaultHeight = 720;
 static const int kDefaultFrameRate = 30;
 static const uint32_t kDefaultMinBitrateBps = 60000;
@@ skipping 181 matching lines @@
   EXPECT_EQ(static_cast<uint32_t>(streams_[0].target_bitrate_bps),
             bitrate_allocation.GetSpatialLayerSum(0));
   EXPECT_EQ(static_cast<uint32_t>(streams_[1].max_bitrate_bps),
             bitrate_allocation.GetSpatialLayerSum(1));
   EXPECT_EQ(kHighScreenshareTl0Bps, bitrate_allocation.GetBitrate(1, 0));
   EXPECT_EQ(kHighScreenshareTl1Bps - kHighScreenshareTl0Bps,
             bitrate_allocation.GetBitrate(1, 1));
 }
 
 }  // namespace webrtc