Adding third_party/openh264 build files for encoding

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2beb66263d6ea3b52f33bffb0aa30978e5aee75d..72c0b872e296d45685322f6e370ee499f656e977 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -549,6 +549,15 @@ 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") {