Previously, there was a race condition in which erasing a draft comment
could occur prior to the updated version of the comment getting saved.
When this happened, if the user replied to the same thread, their old
draft would be pre-populated in the new reply.
This change cancels the 'store' debouncer when erasing a draft comment,
to avoid this race condition.
Bug: Issue 6141
Change-Id: I11a646acf01b1ac986c587e1e8fdb4fd7efc25b7