Create helper functions to use in computeInvariantOutput calls

BUG=skia:

Review URL: https://codereview.chromium.org/643743003
diff --git a/src/gpu/effects/GrConvolutionEffect.h b/src/gpu/effects/GrConvolutionEffect.h
index f61f378..6c55656 100644
--- a/src/gpu/effects/GrConvolutionEffect.h
+++ b/src/gpu/effects/GrConvolutionEffect.h
@@ -100,8 +100,7 @@
     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;