GrFP can express distance vector field req., program builder declares variable for it

src/gpu/GrFragmentProcessor.cpp

Index: src/gpu/GrFragmentProcessor.cpp
diff --git a/src/gpu/GrFragmentProcessor.cpp b/src/gpu/GrFragmentProcessor.cpp
index fdba610356c2dd74a0c786027809a29b0255f7a0..a8a4898e78d99fd7baa3cafbde772f2fc49ac386 100644
--- a/src/gpu/GrFragmentProcessor.cpp
+++ b/src/gpu/GrFragmentProcessor.cpp
@@ -106,6 +106,9 @@ int GrFragmentProcessor::registerChildProcessor(sk_sp<GrFragmentProcessor> child
     if (child->usesLocalCoords()) {
         fUsesLocalCoords = true;
     }
+    if (child->usesDistanceVectorField()) {
+        fUsesDistanceVectorField = true;
+    }
 
     int index = fChildProcessors.count();
     fChildProcessors.push_back(child.release());