Update V8 to r5780 as required by WebKit r71558

Change-Id: Ie3936550b99967a13755930d0dac0a59c3562625
diff --git a/test/mjsunit/string-externalize.js b/test/mjsunit/string-externalize.js
index 5b1f917..da89786 100644
--- a/test/mjsunit/string-externalize.js
+++ b/test/mjsunit/string-externalize.js
@@ -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();