[es6] Implement spec compliant ToPrimitive in the runtime.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index fb6b80dde4ec4c5ffe2a4c8cdff26795384794af..3744ea78d9c7a95524179265b39a871cabf7021b 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -382,12 +382,12 @@ BasicJsonStringifier::Result BasicJsonStringifier::SerializeJSValue(
   if (class_name == isolate_->heap()->String_string()) {
     Handle<Object> value;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
-        isolate_, value, Execution::ToString(isolate_, object), EXCEPTION);
+        isolate_, value, Object::ToString(isolate_, object), EXCEPTION);
     SerializeString(Handle<String>::cast(value));
   } else if (class_name == isolate_->heap()->Number_string()) {
     Handle<Object> value;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
-        isolate_, value, Execution::ToNumber(isolate_, object), EXCEPTION);
+        isolate_, value, Object::ToNumber(isolate_, object), EXCEPTION);
     if (value->IsSmi()) return SerializeSmi(Smi::cast(*value));
     SerializeHeapNumber(Handle<HeapNumber>::cast(value));
   } else if (class_name == isolate_->heap()->Boolean_string()) {