New file api/video/BUILD.gn

Build targets involving files under api/video/ are moved into this
file, from api/BUILD.gn. In addition, drop "_api" part of target
names, and move the header file api/videosinkinterface.h to
api/video/video_sink_interface.h.

Bug: webrtc:9253
Change-Id: I2896d3f063db8dff902bc29738578395b2fcc155
Reviewed-on: https://webrtc-review.googlesource.com/75500
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23207}
diff --git a/api/videosourceinterface.h b/api/videosourceinterface.h
index 065e2dc..17f1604 100644
--- a/api/videosourceinterface.h
+++ b/api/videosourceinterface.h
@@ -14,7 +14,7 @@
 #include <limits>
 
 #include "api/optional.h"
-#include "api/videosinkinterface.h"
+#include "api/video/video_sink_interface.h"
 
 namespace rtc {