[chrome.displaySource] Add WiFi Display session class skeleton and mojo service

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 18bd77509a5c045f587cbf0c1235fd61174b450c..32ed509dc2d1098bc284ceae1e92e8f6704347bc 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -231,6 +231,9 @@ config("feature_flags") {
   if (enable_wayland_server) {
     defines += [ "ENABLE_WAYLAND_SERVER=1" ]
   }
+  if (enable_wifi_display) {
+    defines += [ "ENABLE_WIFI_DISPLAY=1" ]
+  }
   if (proprietary_codecs) {
     defines += [ "USE_PROPRIETARY_CODECS" ]
   }