Add disableStencil method to GrGLGpu

Most the calls to flushStencil were just trying to disable it. This
change cleans up that code so it can just call "disableStencil"
instead of having to muck with GrStencilSettings. It should also make
those calls slightly faster.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2453793002

Review-Url: https://codereview.chromium.org/2453793002
2 files changed