Enable ThreadRestrictionVerifier for StringImpl

third_party/WebKit/Source/wtf/text/StringImplTest.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 71 matching lines @@
   EXPECT_TRUE(
       equal(testStringImpl16.get(),
             StringImpl::create(testCapitalized, 4)->lowerASCII().get()));
 
   static const UChar testWithNonASCII[3] = {0x0061, 0x00e1, 0};  // a\xE1
   static const UChar testWithNonASCIIComparison[3] = {0x0061, 0x00c1,
                                                       0};  // a\xC1
   static const UChar testWithNonASCIICapitalized[3] = {0x0041, 0x00e1,
                                                        0};  // A\xE1
 
+  // Make sure we support RefPtr<const StringImpl>.
+  RefPtr<const StringImpl> constRef = testStringImpl->isolatedCopy();
+  DCHECK(constRef->hasOneRef());
   EXPECT_TRUE(equal(
       StringImpl::create(testWithNonASCII, 2).get(),
       StringImpl::create(testWithNonASCIICapitalized, 2)->lowerASCII().get()));
   EXPECT_FALSE(equal(
       StringImpl::create(testWithNonASCII, 2).get(),
       StringImpl::create(testWithNonASCIIComparison, 2)->lowerASCII().get()));
 }
 
 }  // namespace WTF