[turbofan] Improve JSON output

src/compiler/graph-visualizer.h

Index: src/compiler/graph-visualizer.h
diff --git a/src/compiler/graph-visualizer.h b/src/compiler/graph-visualizer.h
index 3dd66eaf41f3de71182e71902666f3d9f44f3339..611351244beaa5925e0a46a9d7e093e0fe79395e 100644
--- a/src/compiler/graph-visualizer.h
+++ b/src/compiler/graph-visualizer.h
@@ -30,8 +30,9 @@ std::ostream& operator<<(std::ostream& os, const AsDOT& ad);
 
 
 struct AsJSON {
-  explicit AsJSON(const Graph& g) : graph(g) {}
+  AsJSON(const Graph& g, SourcePositionTable* p) : graph(g), positions(p) {}
   const Graph& graph;
+  const SourcePositionTable* positions;
 };
 
 std::ostream& operator<<(std::ostream& os, const AsJSON& ad);