Create helper functions to use in computeInvariantOutput calls

src/gpu/effects/GrRRectEffect.cpp

Index: src/gpu/effects/GrRRectEffect.cpp
diff --git a/src/gpu/effects/GrRRectEffect.cpp b/src/gpu/effects/GrRRectEffect.cpp
index 21135549aca30012e3b780e7f5094db1c27a3bf0..ae1daef858db3cb02b68de09cff5a81b7d9ca943 100644
--- a/src/gpu/effects/GrRRectEffect.cpp
+++ b/src/gpu/effects/GrRRectEffect.cpp
@@ -87,8 +87,7 @@ GrFragmentProcessor* CircularRRectEffect::Create(GrPrimitiveEdgeType edgeType,
 }
 
 void CircularRRectEffect::onComputeInvariantOutput(InvariantOutput* inout) const {
-    inout->fValidFlags = 0;
-    inout->fIsSingleComponent = false;
+    inout->mulByUnknownAlpha();
 }
 
 const GrBackendFragmentProcessorFactory& CircularRRectEffect::getFactory() const {
@@ -426,8 +425,7 @@ EllipticalRRectEffect::Create(GrPrimitiveEdgeType edgeType, const SkRRect& rrect
 }
 
 void EllipticalRRectEffect::onComputeInvariantOutput(InvariantOutput* inout) const {
-    inout->fValidFlags = 0;
-    inout->fIsSingleComponent = false;
+    inout->mulByUnknownAlpha();
 }
 
 const GrBackendFragmentProcessorFactory& EllipticalRRectEffect::getFactory() const {