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

webrtc/DEPS

 # Define rules for which include paths are allowed in our source.
 include_rules = [
   # Base is only used to build Android APK tests and may not be referenced by
   # WebRTC production code.
   "-base",
   "-chromium",
   "+external/webrtc/webrtc",  # Android platform build.
   "+gflags",
   "+libyuv",
   "-webrtc",  # Has to be disabled; otherwise all dirs below will be allowed.
   # Individual headers that will be moved out of here, see webrtc:4243.
   "+webrtc/call.h",
   "+webrtc/common_types.h",
   "+webrtc/config.h",
   "+webrtc/transport.h",
   "+webrtc/typedefs.h",
-  "+webrtc/video_decoder.h",
-  "+webrtc/video_encoder.h",
   "+webrtc/video_frame.h",
   "+webrtc/video_receive_stream.h",
   "+webrtc/video_send_stream.h",
   "+webrtc/voice_engine_configurations.h",
 
   "+WebRTC",
   "+webrtc/api",
   "+webrtc/base",
   "+webrtc/modules/include",
   "+webrtc/test",
@@ skipping 13 matching lines @@
   ],
   "video_receive_stream\.h": [
     "+webrtc/common_video/include",
     "+webrtc/media/base",
   ],
   "video_send_stream\.h": [
     "+webrtc/common_video/include",
     "+webrtc/media/base",
   ],
 }