In discovery code (cast_media_sink_service.cc, dial_media_sink_service_proxy.cc), we do need to migrate to use sequences instead of posting tasks directly to the UI and IO threads.

https://chromium.googlesource.com/chromium/src/+/master/docs/threading_and_tasks.md#Posting-to-the-Main-Thread-or-to-the-IO-Thread-in-the-Browser-Process

To resolve code review comments in: https://chromium-review.googlesource.com/c/575247/