Put ThinStrings behind a flag (off by default)

test/cctest/test-strings.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1178 matching lines @@
   explicit OneByteVectorResource(i::Vector<const char> vector)
       : data_(vector) {}
   virtual ~OneByteVectorResource() {}
   virtual size_t length() const { return data_.length(); }
   virtual const char* data() const { return data_.start(); }
  private:
   i::Vector<const char> data_;
 };
 
 TEST(InternalizeExternal) {
+  FLAG_thin_strings = true;
+  CcTest::InitializeVM();
   i::Isolate* isolate = CcTest::i_isolate();
   Factory* factory = isolate->factory();
   // This won't leak; the external string mechanism will call Dispose() on it.
   OneByteVectorResource* resource =
       new OneByteVectorResource(i::Vector<const char>("prop", 4));
   {
     v8::HandleScope scope(CcTest::isolate());
     v8::Local<v8::String> ext_string =
         v8::String::NewExternalOneByte(CcTest::isolate(), resource)
             .ToLocalChecked();
@@ skipping 453 matching lines @@
   CHECK_EQ(1, CompileRun("external.indexOf('', 1)")
                   ->Int32Value(context.local())
                   .FromJust());
   CHECK_EQ(-1, CompileRun("external.indexOf('a', 1)")
                    ->Int32Value(context.local())
                    .FromJust());
   CHECK_EQ(-1, CompileRun("external.indexOf('$')")
                    ->Int32Value(context.local())
                    .FromJust());
 }