Refactor code for standardizing old gradient args into a standalone function.
Refactor code for standardizing old gradient args into a standalone function.

The new function is defined inline in its caller, for the moment, to minimize diffs; a later commit will move it elsewhere.

webcompat/css-fixme,hallvors/css-fixme,hallvors/css-fixme,Kuniwak/css-fixme,webcompat/css-fixme