Those calls deal with the GTimer used to keep information about download elapsed time, but the timer may never get initialized if _start is not called. If nothing handles the download-requested signal, for example, this will emit nasty warning/critical messages in the terminal.
Created attachment 28343 [details] proposed fix
Comment on attachment 28343 [details] proposed fix r=me. "existence" is misspelt in the ChangeLog, the comma is unnecessary there too, and perhaps the comments in the code could be a little more consistent (exists vs non-NULL).
Landed as r41478.