Separate i420 and i444 implementations to separate targets.

This means we can properly declare the dependency between
libjingle_peerconnection_api and video_frame_api. i420
pulls in system_wrappers, which can't be a dependency of
the public API.

Plan:
1) Land this CL + send out PSA
2) Make all direct users of i420_buffer depend on the
   new video_frame_api_i420 target
3) Move i420_buffer.cc to the new target
4) Make libjingle_peerconnection_api depend on
   video_frame_api, since it no longer contains i420 code

Bug: webrtc:7504
Change-Id: I30d90f2ac7af53748859bbde8aed92386d5501f9
Reviewed-on: https://webrtc-review.googlesource.com/9382
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20656}
diff --git a/modules/BUILD.gn b/modules/BUILD.gn
index 5d4987e..d534f56 100644
--- a/modules/BUILD.gn
+++ b/modules/BUILD.gn
@@ -37,6 +37,7 @@
     "..:webrtc_common",
     "../api:optional",
     "../api:video_frame_api",
+    "../api:video_frame_api_i420",
     "../rtc_base:rtc_base_approved",
     "video_coding:codec_globals_headers",
   ]