Adding third_party/openh264 build files for encoding

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f160eb2f08528c931cb3699a02712721adb03e89..60a313e750e6eb8c073ed668d1d85f885a16e883 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,6 +13,7 @@ import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build_overrides/v8.gni")
 import("//media/media_options.gni")
+import("//third_party/openh264/openh264_args.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
@@ -587,6 +588,14 @@ group("both_gn_and_gyp") {
   if (is_ios || is_mac) {
     deps += [ "//media/cast:cast_h264_vt_encoder_unittests" ]
   }
+
+  if (use_openh264) {
+    deps += [
+      "//third_party/openh264:common",
+      "//third_party/openh264:encoder",
+      "//third_party/openh264:processing",
+    ]
+  }
 }
 
 group("gn_only") {