Replace unprefixed `box-sizing` property with mixin
Replace unprefixed `box-sizing` property with mixin

Replacing unprefixed `box-sizing` property with corresponding mixin.

Change-Id: Ie4030fa6b78ab7051a2d6d14efe2233875cfc2ee

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui