Define the color etc. in the base styles, so that other drawers will
not have to redefine them to display the shield.
Bug: T214049
Change-Id: Ife957374cb0d21446db2067171e68fb726ad8347
* truncated-text is only used in MainMenu, last modified bar,
a few special pages
Making this a mixin and limiting it to mobile.startup skinStyles
and mobile.special.styles thus makes sense
* cloaked-element is only ever added by Skin.js in mobile.startup
* position-fixed is only ever added by Skin.js
Change-Id: Iaf7061e9dda87eb25c963f8a281175af08f358e5