Replace `sendRequest()` calls with `sendMessage()` ones in marks.coffee

Fixing my incomplete commit 9403692e51, with moving from old `sendRequest` API to the new `sendMessage` in two more files: `marks.coffee`, both background and content scripts.

I'm sorry, I don't know how I could possibly miss these four calls.
Thanks for spotting this, @harry313!

So, merging this change will resolve #849.