Proof of concept using libvpx's y4minput.c

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 62e06ad4ac9804dd364ca41f81dd531d3f738daa..1f8e0d891c84f0b1e03e152afb6c81a2a60ad88d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -19,3 +19,14 @@ group("default") {
     deps += [ "//webrtc:webrtc_tests" ]
   }
 }
+
+# This needs to be on the top-level to avoid violating the PRESUBMIT rule that
+# disallows including sources in higher-level directory levels.
+rtc_source_set("libvpx_y4minput") {
+  sources = [
+    "third_party/libvpx/source/libvpx/y4minput.c",
+  ]
+  deps = [
+    "//third_party/libvpx",
+  ]
+}