Avoid CSSJanus to flip the box-shadow property of the message list

A noflip annotation has been added to avoid CSSJanus to affect the
values of the shadow used in the message list header when a right-to-left language
is used. A screenshot of the issue can be viewed at http://i.imgur.com/tMhEr.png

Change-Id: Idc8ad228464a18d95617d9ddc75c0fa107f73c07