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

webrtc/api/video_codecs/BUILD.gn

-# 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.
-
-import("../../webrtc.gni")
-if (is_android) {
-  import("//build/config/android/config.gni")
-  import("//build/config/android/rules.gni")
-}
-
-rtc_source_set("video_codecs_api") {
-  sources = [
-    "video_decoder.h",
-    "video_encoder.h",
-  ]
-
-  deps = [
-    # TODO(ilnik): Add dependency on webrtc/video_frame.h when it will have it's
-    # own build target.
-
-    "../..:webrtc_common",
-    "../../base:rtc_base_approved",
-  ]
-}