Create helper functions to use in computeInvariantOutput calls

src/gpu/effects/GrMatrixConvolutionEffect.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrMatrixConvolutionEffect_DEFINED
 #define GrMatrixConvolutionEffect_DEFINED
 
@@ skipping 63 matching lines @@
                               SkScalar gain,
                               SkScalar bias,
                               const SkIPoint& kernelOffset,
                               GrTextureDomain::Mode tileMode,
                               bool convolveAlpha);
 
     virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE;
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
         // TODO: Try to do better?
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownColor();
     }
 
     SkIRect         fBounds;
     SkISize         fKernelSize;
     float           fKernel[MAX_KERNEL_SIZE];
     float           fGain;
     float           fBias;
     float           fKernelOffset[2];
     bool            fConvolveAlpha;
     GrTextureDomain fDomain;
 
     GR_DECLARE_FRAGMENT_PROCESSOR_TEST;
 
     typedef GrSingleTextureEffect INHERITED;
 };
 
 #endif