Update V8 to version 3.20.15.5

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22891003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@217063 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": "16142",
+  "v8_revision": "16159",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling WebRTC
   # and V8 without interference from each other.