Count external string resources that were assigned to symbol strings.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index be7d73f50906edcf0343d326e592520b71195285..165447dcf39932e0f31b88c2b3bd3eeed4af1558 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4900,7 +4900,7 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
   }
   CHECK(resource && resource->data());
   bool result = obj->MakeExternal(resource);
-  if (result && !obj->IsSymbol()) {
+  if (result) {
     isolate->heap()->external_string_table()->AddString(*obj);
   }
   return result;
@@ -4937,7 +4937,7 @@ bool v8::String::MakeExternal(
   }
   CHECK(resource && resource->data());
   bool result = obj->MakeExternal(resource);
-  if (result && !obj->IsSymbol()) {
+  if (result) {
     isolate->heap()->external_string_table()->AddString(*obj);
   }
   return result;