Fix flushing of HWAA state relating to mixed samples

Fixes the sense of the "stencilEnabled" argument sent to
flushHWAAState.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4872

Change-Id: If7097f2819607864c95b0ffd04dc1d302fa66fdc
Reviewed-on: https://skia-review.googlesource.com/4872
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed