[GN] Add JSON project writer

tools/gn/visibility.h

Index: tools/gn/visibility.h
diff --git a/tools/gn/visibility.h b/tools/gn/visibility.h
index d6d7f1ffe01a2796b0d1df8b99c1ccc617721047..c9b27c8717a83f9b7bf657e4568e42938821dff0 100644
--- a/tools/gn/visibility.h
+++ b/tools/gn/visibility.h
@@ -11,6 +11,10 @@
 #include "tools/gn/label_pattern.h"
 #include "tools/gn/source_dir.h"
 
+namespace base {
+class Value;
+}
+
 class Err;
 class Item;
 class Label;
@@ -43,6 +47,9 @@ class Visibility {
   // result will end in a newline.
   std::string Describe(int indent, bool include_brackets) const;
 
+  // Returns value representation of this visibility
+  std::unique_ptr<base::Value> AsValue() const;
+
   // Helper function to check visibility between the given two items. If
   // to is invisible to from, returns false and sets the error.
   static bool CheckItemVisibility(const Item* from, const Item* to, Err* err);