[TurboFan]: Add JSON output for the visualizer

src/compiler/graph-visualizer.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_GRAPH_VISUALIZER_H_
 #define V8_COMPILER_GRAPH_VISUALIZER_H_
 
 #include "src/v8.h"
 
 namespace v8 {
 namespace internal {
 
 class OStream;
 
 namespace compiler {
 
 class Graph;
 
 struct AsDOT {
   explicit AsDOT(const Graph& g) : graph(g) {}
   const Graph& graph;
 };
 
 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
 
 #endif  // V8_COMPILER_GRAPH_VISUALIZER_H_