Bug 1044960 - hide startscreen if tab url was null
Bug 1044960 - fix lint error, r=me
Bug 1047475 - [Messages] The suggestions panel scroll position should be reset after it's been hidden

cherry-picked from 20eeb7c7724ca51a2139cdb7bd5dd137f4be830f
Merge pull request #23402 from luke-chang/1047475_v2.0_sms_reset_scrollbar

Bug 1047475 - [Messages] The suggestions panel scroll position should be reset after it's been hidden