Create helper functions to use in computeInvariantOutput calls

src/effects/SkLumaColorFilter.cpp

Index: src/effects/SkLumaColorFilter.cpp
diff --git a/src/effects/SkLumaColorFilter.cpp b/src/effects/SkLumaColorFilter.cpp
index 1ca3c1623a0171e06e596a8af356b9e8051b2094..5868f4c7c83b64f3e844fe1d97d877f218ac0dac 100644
--- a/src/effects/SkLumaColorFilter.cpp
+++ b/src/effects/SkLumaColorFilter.cpp
@@ -114,10 +114,8 @@ private:
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        // The output is always black.
-        inout->fColor = GrColorPackRGBA(0, 0, 0, GrColorUnpackA(inout->fColor));
-        inout->fValidFlags = kRGB_GrColorComponentFlags;
-        inout->fIsSingleComponent = false;
+        // The output is always black. The alpha value for the color passed in is arbitrary.
+        inout->setToOther(kRGB_GrColorComponentFlags, GrColorPackRGBA(0, 0, 0, 0));
     }
 };