Update V8 to version 3.12.18.

R=jkummerow@chromium.org


Review URL: https://chromiumcodereview.appspot.com/10824083

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149003 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -61,7 +61,7 @@ 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": "12207",
+  "v8_revision": "12227",
   "webrtc_revision": "2529",
   "jsoncpp_revision": "248",
   "nss_revision": "145873",