Add untracked headers to video_coding.

This creates a new target for pure defines and interfaces. I think
that makes sense (though include/ makes it harder to see when .cc and
.h files should live together).

Bug: webrtc:7620
Change-Id: Ifb0f50faf99166202836c0446feed3443eb52c6e
Reviewed-on: https://webrtc-review.googlesource.com/34657
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21516}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index ab49d11..3c4f18e 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -67,6 +67,7 @@
     "../call:rtp_interfaces",
     "../call:video_stream_api",
     "../modules/rtp_rtcp:rtp_rtcp_format",
+    "../modules/video_coding:video_codec_interface",
     "../rtc_base:checks",
     "../system_wrappers:field_trial_api",
     "../system_wrappers:metrics_api",
@@ -308,6 +309,7 @@
       "../modules/rtp_rtcp:rtp_rtcp_format",
       "../modules/utility",
       "../modules/video_coding",
+      "../modules/video_coding:video_codec_interface",
       "../modules/video_coding:video_coding_utility",
       "../modules/video_coding:webrtc_h264",
       "../modules/video_coding:webrtc_vp8",