Adding third_party/openh264 build files for encoding

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f58231fc4d16d6b9cdbed524e920c349566a2ae1..fc445a8c006812dd7c14bcafe8575d15672abfb2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -586,6 +586,15 @@ group("both_gn_and_gyp") {
   if (is_chromecast) {
     deps += [ "//chromecast:cast_shell" ]
   }
+
+  if (use_openh264) {
+    deps += [
+      "//third_party/openh264:openh264_common",
+      "//third_party/openh264:openh264_processing",
+      "//third_party/openh264:openh264_encoder",
+      "//third_party/openh264:openh264_decoder",
+    ]
+  }
 }
 
 group("gn_only") {