Added ability to lock strings to prevent their representation or encoding from changing.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index ed87fa0ce1ea46edd8add709b8c864842fac064b..6a58417879c438d84d85ae26424f6c421e5b395d 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4479,6 +4479,9 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
     return false;
   }
+  if (isolate->heap()->IsStringLocked(*obj)) {
+    return false;
+  }
   if (isolate->heap()->IsInGCPostProcessing()) {
     return false;
   }