extend SkShader to report a luminance-color to be used for gamma correction

src/effects/gradients/SkGradientShaderPriv.h

Index: src/effects/gradients/SkGradientShaderPriv.h
diff --git a/src/effects/gradients/SkGradientShaderPriv.h b/src/effects/gradients/SkGradientShaderPriv.h
index 1787e243886068cb31154d2ab21d7399a603c5c6..da50da6d53af5385c44713f360f18641b03df93a 100644
--- a/src/effects/gradients/SkGradientShaderPriv.h
+++ b/src/effects/gradients/SkGradientShaderPriv.h
@@ -214,6 +214,8 @@ protected:
 
     void commonAsAGradient(GradientInfo*, bool flipGrad = false) const;
 
+    virtual bool onAsLuminanceColor(SkColor*) const SK_OVERRIDE;
+
     /*
      * Takes in pointers to gradient color and Rec info as colorSrc and recSrc respectively.
      * Count is the number of colors in the gradient