Create helper functions to use in computeInvariantOutput calls

src/gpu/GrAAConvexPathRenderer.cpp

Index: src/gpu/GrAAConvexPathRenderer.cpp
diff --git a/src/gpu/GrAAConvexPathRenderer.cpp b/src/gpu/GrAAConvexPathRenderer.cpp
index 550d7ae79909d9e32362e430e1a8e0f57f3ccbbf..9e16177f310084bb74332b1a1d7fe8e586a3c4b8 100644
--- a/src/gpu/GrAAConvexPathRenderer.cpp
+++ b/src/gpu/GrAAConvexPathRenderer.cpp
@@ -589,8 +589,7 @@ private:
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     const GrShaderVar& fInQuadEdge;