[json] move json parser and stringifier into own compilation units.

src/runtime/runtime.h

Index: src/runtime/runtime.h
diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index 561353b49493aa4c87e7412124a2d9dd933d4e9e..7397575a37fe409400019fe9915415a7007a3686 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -1089,12 +1089,6 @@ class Runtime : public AllStatic {
   MUST_USE_RESULT static MaybeHandle<Object> GetObjectProperty(
       Isolate* isolate, Handle<Object> object, Handle<Object> key);
 
-  MUST_USE_RESULT static MaybeHandle<Object> BasicJsonStringify(
-      Isolate* isolate, Handle<Object> object, Handle<String> gap);
-
-  MUST_USE_RESULT static MaybeHandle<Object> BasicJsonStringifyString(
-      Isolate* isolate, Handle<String> string);
-
   enum TypedArrayId {
     // arrayIds below should be synchronized with typedarray.js natives.
     ARRAY_ID_UINT8 = 1,