Merge pull request #10151 from ian-liu/mms/Bug877615_recipients_do_not_clear_after_discarding_a_previous_multi_recipients_message

Bug 877615 - [MMS] New message composer: Recipients field stays expanded after discarding a previous multi recipient message, r=@steveck-chung(cherry picked from commit 61440e3abaf2d94aa4146c9638ec9f99ab19be3e)