Make sure to discard a draft whenever we're saving.

Release-Notes: fixes deletion of patchset comment right before saving.
Google-Bug-Id: b/309970044, b/309533366
Change-Id: I420fd0193f4911aca85b26989ebfa60f784f90d7
(cherry picked from commit 063fa5047dac347232e503ae8cc5b3aa15e68bdc)