Dawn: fix single-sided stencil.
Dawn: fix single-sided stencil.

GrStencilSettings requires that single-sided stencil be accessed via
frontAndBack(), not via front() or back().

Change-Id: Ib35e6187bda05e21c164685f7b726ce4849979fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249416
Reviewed-by: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>
Commit-Queue: Stephen White <30b0ee0ef1651e8ba59cfd41f16e23abfe9aca8a@chromium.org>

aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia