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",
+  ]
+}