Ensure strings are never externalized as a side-effect of doing a GC.

src/api.cc

Index: src/api.cc
===================================================================
--- src/api.cc	(revision 8139)
+++ src/api.cc	(working copy)
@@ -4264,6 +4264,9 @@
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
     return false;
   }
+  if (isolate->heap()->gc_post_process_count() > 0) {
+    return false;
+  }
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);
@@ -4296,6 +4299,9 @@
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
     return false;
   }
+  if (isolate->heap()->gc_post_process_count() > 0) {
+    return false;
+  }
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);