Replaces the searchfield-cancel icon with the close-12 icon so that we
can style the background-color of the icon as needed.
Since we need close-12.svg to be content accessible, I needed to move
the icon to layout/style/res and add it as a content accessible file.
This resulted in replacing the existing uses of
chrome://global/skin/icons/close-12.svg with the new resource URL,
resource://content-accessible/close-12.svg.
Differential Revision: https://phabricator.services.mozilla.com/D243564
134dc6c03a