Create helper functions to use in computeInvariantOutput calls

src/gpu/effects/GrConvolutionEffect.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrConvolutionEffect_DEFINED
 #define GrConvolutionEffect_DEFINED
 
@@ skipping 82 matching lines @@
                         int halfWidth,
                         float gaussianSigma,
                         bool useBounds,
                         float bounds[2]);
 
     virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE;
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const {
         // If the texture was opaque we could know that the output color if we knew the sum of the
         // kernel values.
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownColor();
     }
 
     GR_DECLARE_FRAGMENT_PROCESSOR_TEST;
 
     typedef Gr1DKernelEffect INHERITED;
 };
 
 #endif