Media Remoting: Add RemotingController.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index d6ab4f4249fdf03b06e294cc4f537fe282e66ab1..61dac760ac3aad5cda26b90799147d60ad743515 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -119,3 +119,10 @@ declare_args() {
     }
   }
 }
+
+declare_args() {
+  # This switch defines whether the Media Remoting implementation will be built.
+  # When enabled, media is allowed to be renderer and played back on remote
+  # devices when the tab is being casted and other conditions are met.
+  enable_media_remoting = !is_chromecast && !is_ios && !is_android
+}