Replace fWillReadFragmentPosition with a bitfield

src/gpu/GrPipeline.h

Index: src/gpu/GrPipeline.h
diff --git a/src/gpu/GrPipeline.h b/src/gpu/GrPipeline.h
index 4f81ae40d0276223f2de88d4edd3238ea3576ebf..3b692943fc2b98495fd962b8e29ec7e439f8ed81 100644
--- a/src/gpu/GrPipeline.h
+++ b/src/gpu/GrPipeline.h
@@ -160,7 +160,10 @@ public:
 
     ///////////////////////////////////////////////////////////////////////////
 
-    bool readsFragPosition() const { return fReadsFragPosition; }
+    bool readsFragPosition() const {
+        return fBuiltInState & GrProcessor::kFragmentPosition_BuiltInState;
+    }
+
     bool ignoresCoverage() const { return fIgnoresCoverage; }
 
 private:
@@ -200,7 +203,7 @@ private:
     uint32_t                            fFlags;
     ProgramXferProcessor                fXferProcessor;
     FragmentProcessorArray              fFragmentProcessors;
-    bool                                fReadsFragPosition;
+    GrProcessor::BuiltInState           fBuiltInState;
     bool                                fIgnoresCoverage;
 
     // This value is also the index in fFragmentProcessors where coverage processors begin.