Simplify MSAA path renderer

This simplification is based on the observations that:
   there were never more than 2 passes
   the only time passes[] was null was the single-pass, !stencilOnly case
   only kBoth_DrawFace was was ever used

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2135053002

Review-Url: https://codereview.chromium.org/2135053002
1 file changed