Create helper functions to use in computeInvariantOutput calls

src/effects/SkPerlinNoiseShader.cpp

Index: src/effects/SkPerlinNoiseShader.cpp
diff --git a/src/effects/SkPerlinNoiseShader.cpp b/src/effects/SkPerlinNoiseShader.cpp
index e9f6c60da8eac52c2c8d5206b73a49279eadf218..3301886d452029893066c2261df60f5c1ce01484 100644
--- a/src/effects/SkPerlinNoiseShader.cpp
+++ b/src/effects/SkPerlinNoiseShader.cpp
@@ -587,8 +587,7 @@ private:
     }
 
     void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0; // This is noise. Nothing is constant.
-        inout->fIsSingleComponent = false;
+        inout->unknownOutput();

[bsalomon, 2014/10/10 19:48:16]

does this generate a random alpha?

[egdaniel, 2014/10/13 16:44:57]

It produces a random alpha and returns a vec4 color with each component
multiplied by alpha. It ignores inputColor.

     }
 
     GrPerlinNoiseEffect(SkPerlinNoiseShader::Type type,