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/sdk/BUILD.gn b/sdk/BUILD.gn
index b62dd9b..655c79f 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -525,6 +525,7 @@
         "../common_video",
         "../media:rtc_media_base",
         "../modules:module_api",
+        "../modules/video_coding:video_codec_interface",
         "../pc:peerconnection",
         "../rtc_base:checks",
         "../rtc_base:rtc_base",
@@ -640,6 +641,7 @@
           "../../system_wrappers:system_wrappers_default",
           "../media:rtc_media_base",
           "../modules:module_api",
+          "../modules/video_coding:video_codec_interface",
           "../rtc_base:rtc_base_tests_utils",
           "//test:test_support",
           "//third_party/ocmock",
@@ -844,6 +846,7 @@
         "../media:rtc_media",
         "../media:rtc_media_base",
         "../modules:module_api",
+        "../modules/video_coding:video_codec_interface",
         "../modules/video_coding:video_coding_utility",
         "../modules/video_coding:webrtc_h264",
         "../rtc_base:checks",