Make ExternalizeStringExtension free its data on exit to please

test/mjsunit/string-externalize.js

Index: test/mjsunit/string-externalize.js
===================================================================
--- test/mjsunit/string-externalize.js	(revision 5717)
+++ test/mjsunit/string-externalize.js	(working copy)
@@ -25,7 +25,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // 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;
 
@@ -93,3 +93,7 @@
 for (var i = 0; i < 10; i++) {
   test();
 }
+
+// Clean up string to make Valgrind happy.
+gc();
+gc();