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 bad7ebe0360cd046262d39b4fa639aeecb874445..df2ccf0480ab62d7b9e4d492282092acead63d2a 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());