With this change, toast messages show when diff comments are being saved
and notify when saving is complete, even when the diff view is no-longer
loaded. This signal is intended to encourage users to write diff
comments and feel free to navigate away to view other parts of a change.
With this change, new toast messages are able to replace existing toast
messages.
Feature: Issue 6970
Change-Id: I5973673d85c73b14794cb5e5b21327dd0c27ec1d
494e7d42b7