Refactor code for standardizing old gradient args

[dholbert]

This patch-stack refactors the existing code for standardizing old gradient args into a helper-function.

• The first patch creates the function (inline in the caller for now, to minimize bitrot, so you can see what's being changed).
• The second patch adjusts the function so that its "args" array no longer expects to have the gradient type (linear/radial) in the 0th slot. (This is needed for https://bugzilla.mozilla.org/show_bug.cgi?id=1132748 )
• The third patch de-indents the function. (whitespace-only)
• The fourth patch moves the function out of its caller, to be a top-level function. (code-move only)

[dholbert]

I sanity-checked the final cssfixme.htm with this style (exercising both a linear & radial gradient):

div {
  width: 100px;
  height: 100px;
}
.foo {
  background: -webkit-gradient(linear, left top, right bottom, from(white), to(black));
}
.bar {
  background: -webkit-gradient(radial, center center, 0, center center, 50, from(yellow), color-stop(20%, orange), color-stop(40%, red), color-stop(60%, green), color-stop(80%, blue), to(purple));
}

It's able to successfully unprefix (& the unprefixed style produces the same rendering in Firefox as the prefixed rendering produces in Chrome).