Fix GrDrawContextPriv::stencilRect

Updates stencilRect to not call getFillRectBatch. This method can
return a batch that uses coverage AA, which isn't appropriate for
stencil draws.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2057613003

Review-Url: https://codereview.chromium.org/2057613003
3 files changed