Update V8 to version 3.21.16.1

R=machenbach@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23480080

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223586 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -34,7 +34,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and V8 without interference from each other.
-  "v8_revision": "16723",
+  "v8_revision": "16747",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling WebRTC
   # and V8 without interference from each other.