Put ThinStrings behind a flag (off by default)

test/cctest/test-strings.cc

Index: test/cctest/test-strings.cc
diff --git a/test/cctest/test-strings.cc b/test/cctest/test-strings.cc
index ff3b7bddaa094edf0e5b68ad15a420378838b74f..e4df339d8b62e92852c4d80ddbce530dd62cf656 100644
--- a/test/cctest/test-strings.cc
+++ b/test/cctest/test-strings.cc
@@ -1196,6 +1196,8 @@ class OneByteVectorResource : public v8::String::ExternalOneByteStringResource {
 };
 
 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.