[TurboFan]: Add JSON output for the visualizer

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 12532bacf8bbbe4c1f6d3f15c80a177ba1f7bfdf..db92dc2a30315759a4d658bd812f00f7223f45e0 100644
--- a/src/compiler/graph-visualizer.h
+++ b/src/compiler/graph-visualizer.h
@@ -22,6 +22,13 @@ struct AsDOT {
 };
 
 OStream& operator<<(OStream& os, const AsDOT& ad);
+
+struct AsJSON {
+  explicit AsJSON(const Graph& g) : graph(g) {}
+  const Graph& graph;
+};
+
+OStream& operator<<(OStream& os, const AsJSON& ad);
 }
 }
 }  // namespace v8::internal::compiler