(Some) gradients are gamma and gamut correct on GPU

include/gpu/GrColorSpaceXform.h

Index: include/gpu/GrColorSpaceXform.h
diff --git a/include/gpu/GrColorSpaceXform.h b/include/gpu/GrColorSpaceXform.h
index 2d2cc867a59ce30ddc2bce8d835136533f5bc67b..7c88c628507c73380626a4172cafeb47d8e633b7 100644
--- a/include/gpu/GrColorSpaceXform.h
+++ b/include/gpu/GrColorSpaceXform.h
@@ -23,7 +23,7 @@ public:
 
     static sk_sp<GrColorSpaceXform> Make(SkColorSpace* src, SkColorSpace* dst);
 
-    const SkMatrix44& srcToDst() { return fSrcToDst; }
+    const SkMatrix44& srcToDst() const { return fSrcToDst; }
 
     /**
      * GrGLSLFragmentProcessor::GenKey() must call this and include the returned value in its
@@ -34,6 +34,8 @@ public:
         return SkToBool(xform) ? 1 : 0;
     }
 
+    static bool Equals(const GrColorSpaceXform* a, const GrColorSpaceXform* b);
+
     GrColor4f apply(const GrColor4f& srcColor);
 
 private: