Some mask-* property related cleanups

 * Remove "Webkit" from some function names to prefer the
   unprefixed names.

 * Simplify SVGMaskPainter by folding PaintSVGMask(), PaintMaskLayers()
   and (one instance of) ResolveElementReference().

Change-Id: Ib8283f1945fb1930d6e95b6f60c30c1276f0e948
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401800


Auto-Submit: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280001}