Bug 950074 - [Messages] The "you received a message" notice has a bad layout r=schung

For some reason the commited CSS was wrong.
Converted it to flex box so that it looks correct also on bigger screens (like
the Peak).
Bug 950498. Update will-animate to will-change: scroll-position
Bug924170 - [Video] - Add Marionette tests for Video App r=jugglinmike
Merge pull request #14669 from julienw/950074-fix-new-message-notice

Bug 950074 - [Messages] The "you received a message" notice has a bad la...
Merge pull request #12717 from punamdahiya/Bug924170_Video_Marionette_Tests

Bug924170 - [Video] - Add Marionette tests for Video App r=jugglinmike
Merge pull request #14680 from changm/bug950498

Bug 950498. Update will-animate to will-change: scroll-position