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

webrtc/modules/video_coding/codecs/vp8/simulcast_rate_allocator.h

 /*
  *  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.
  */
 
 #ifndef WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_RATE_ALLOCATOR_H_
 #define WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_RATE_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/common_video/include/video_bitrate_allocator.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
 class SimulcastRateAllocator : public VideoBitrateAllocator,
                                public TemporalLayersListener {
  public:
   explicit SimulcastRateAllocator(
       const VideoCodec& codec,
       std::unique_ptr<TemporalLayersFactory> tl_factory);
 
   void OnTemporalLayersCreated(int simulcast_id,
                                TemporalLayers* layers) override;
 
   BitrateAllocation GetAllocation(uint32_t total_bitrate_bps,
                                   uint32_t framerate) override;
   uint32_t GetPreferredBitrateBps(uint32_t framerate) override;
   const VideoCodec& GetCodec() const;
 
  private:
   const VideoCodec codec_;
   std::map<uint32_t, TemporalLayers*> temporal_layers_;
   std::unique_ptr<TemporalLayersFactory> tl_factory_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(SimulcastRateAllocator);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_RATE_ALLOCATOR_H_