Move code related to VideoCodingModule to its own build target

The new target, modules/video_coding:video_coding_legacy, is not
depended upon by any webrtc non-test code.

Bug: webrtc:7408
Change-Id: I94127e2b8b3b8f15917bfa38e602f8face91fcdb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152163
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29133}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index e4925a2..c0b9435 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -223,6 +223,7 @@
     "../rtc_base/synchronization:sequence_checker",
     "../rtc_base/system:fallthrough",
     "../rtc_base/task_utils:repeating_task",
+    "../system_wrappers",
     "../system_wrappers:field_trial",
     "//third_party/abseil-cpp/absl/algorithm:container",
     "//third_party/abseil-cpp/absl/memory",