Create helper functions to use in computeInvariantOutput calls

src/gpu/GrAARectRenderer.cpp

Index: src/gpu/GrAARectRenderer.cpp
diff --git a/src/gpu/GrAARectRenderer.cpp b/src/gpu/GrAARectRenderer.cpp
index 88b0306a53fb9fcca3c1a2115c962c4f24d0ebeb..1a144d9b7591b19f0e0ce3aa4bed289105e6696e 100644
--- a/src/gpu/GrAARectRenderer.cpp
+++ b/src/gpu/GrAARectRenderer.cpp
@@ -108,8 +108,7 @@ private:
     virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE { return true; }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     GR_DECLARE_GEOMETRY_PROCESSOR_TEST;
@@ -253,8 +252,7 @@ private:
     virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE { return true; }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     const GrShaderVar& fInRectEdge;