Make ExternalizeStringExtension free its data on exit to please

test/mjsunit/string-externalize.js

 // Copyright 2010 the V8 project authors. All rights reserved.
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 // 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.
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 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Flags: --expose-externalize-string
+// Flags: --expose-externalize-string --expose-gc
 
 var size = 1024;
 
 function test() {
   var str = "";
 
   // Build an ascii cons string.
   for (var i = 0; i < size; i++) {
       str += String.fromCharCode(i & 0x7f);
   }
@@ skipping 47 matching lines @@
   }
 
   // Flattened string should still be two-byte.
   assertFalse(isAsciiString(str2));
 }
 
 // Run the test many times to ensure IC-s don't break things.
 for (var i = 0; i < 10; i++) {
   test();
 }
+
+// Clean up string to make Valgrind happy.
+gc();
+gc();