Recommit introducing short external strings.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 1a47e096fb7b097a9eae7f7d0351feebbd15198d..d3a6f0b06878e9439c5162cfe256e13adfccbe75 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4556,12 +4556,9 @@ bool v8::String::CanMakeExternal() {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
   if (IsDeadCheck(isolate, "v8::String::CanMakeExternal()")) return false;
-  if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
-    return false;
-  }
+  if (isolate->string_tracker()->IsFreshUnusedString(obj)) return false;
   int size = obj->Size();  // Byte size of the original string.
-  if (size < i::ExternalString::kSize)
-    return false;
+  if (size < i::ExternalString::kShortSize) return false;
   i::StringShape shape(*obj);
   return !shape.IsExternal();
 }