Add query to GrXPFactory about coverage-as-alpha optimization

This will be needed to have GrDrawOps that haven't yet built pipelines.

Change-Id: If5292aaa5dc9f98dccbe27be98960b630332158d
Reviewed-on: https://skia-review.googlesource.com/9480
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/effects/GrCoverageSetOpXP.h b/src/gpu/effects/GrCoverageSetOpXP.h
index 02db11b..807a2b0 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.h
+++ b/src/gpu/effects/GrCoverageSetOpXP.h
@@ -45,6 +45,8 @@
         return false;
     }
 
+    bool compatibleWithCoverageAsAlpha(bool colorIsOpaque) const override { return false; }
+
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkRegion::Op fRegionOp;