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>
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 3e98cab..ad5fcf7 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -2064,7 +2064,7 @@
     this->flushStencil(stencil);
     this->flushScissor(pipeline.getScissorState(), glRT->getViewport(), glRT->origin());
     this->flushWindowRectangles(pipeline.getWindowRectsState(), glRT);
-    this->flushHWAAState(glRT, pipeline.isHWAntialiasState(), stencil.isDisabled());
+    this->flushHWAAState(glRT, pipeline.isHWAntialiasState(), !stencil.isDisabled());
 
     // This must come after textures are flushed because a texture may need
     // to be msaa-resolved (which will modify bound FBO state).