Replace fWillReadFragmentPosition with a bitfield

src/effects/SkBlurMaskFilter.cpp

Index: src/effects/SkBlurMaskFilter.cpp
diff --git a/src/effects/SkBlurMaskFilter.cpp b/src/effects/SkBlurMaskFilter.cpp
index 6679e8aa414bf70b5c26f1554d9f652f9aa0ee9e..5c502996acdd14a24e8cb839c3a5c81a4f59042f 100644
--- a/src/effects/SkBlurMaskFilter.cpp
+++ b/src/effects/SkBlurMaskFilter.cpp
@@ -811,7 +811,7 @@ GrRectBlurEffect::GrRectBlurEffect(const SkRect& rect, float sigma, GrTexture *b
     , fPrecision(precision) {
     this->initClassID<GrRectBlurEffect>();
     this->addTextureAccess(&fBlurProfileAccess);
-    this->setWillReadFragmentPosition();
+    this->enableBuiltInState(kFragmentPosition_BuiltInState);
 }
 
 void GrRectBlurEffect::onGetGLSLProcessorKey(const GrGLSLCaps& caps,
@@ -1019,7 +1019,7 @@ GrRRectBlurEffect::GrRRectBlurEffect(float sigma, const SkRRect& rrect, GrTextur
       fNinePatchAccess(ninePatchTexture) {
     this->initClassID<GrRRectBlurEffect>();
     this->addTextureAccess(&fNinePatchAccess);
-    this->setWillReadFragmentPosition();
+    this->enableBuiltInState(kFragmentPosition_BuiltInState);
 }
 
 bool GrRRectBlurEffect::onIsEqual(const GrFragmentProcessor& other) const {