Update V8 to version 3.4.12.1.

DEPS

 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   "googlecode_url": "http://%s.googlecode.com/svn",
   "webkit_trunk": "http://svn.webkit.org/repository/webkit/trunk",
   "nacl_trunk": "http://src.chromium.org/native_client/trunk",
   "webkit_revision": "90896",
   "chromium_git": "http://git.chromium.org/git",
   "swig_revision": "69281",
   # These hashes need to be updated when nacl_revision is changed.
   # After changing nacl_revision, run 'gclient runhooks' to get the new values.
   "nacl_irt_hash_x86_32": "40737ff789c03561a79dc7802f16e84d4dbe3b07",
   "nacl_irt_hash_x86_64": "8d7d2f6f5ef19467981eb89ab2a138e79aa5926d",
   "nacl_revision": "5988",  # If you update this, change ppapi below.
   "nacl_tools_revision": "5360",  # tools_rev in native_client/DEPS
   "libjingle_revision": "71",
   "libvpx_revision": "90416",
   "ffmpeg_revision": "90064",
   "skia_revision": "1832",
-  "v8_revision": "8592",
+  "v8_revision": "8640",
   "webrtc_revision": "90",
 }
 
 deps = {
   "src/breakpad/src":
     (Var("googlecode_url") % "google-breakpad") + "/trunk/src@786",
 
   "src/build/util/support":
     "/trunk/deps/support@20411",
 
@@ skipping 349 matching lines @@
   {
     # This downloads binaries for Native Client's integrated runtime (IRT)
     # library, which is built as NaCl untrusted code.
     "pattern": ".",
     "action": ["python", "src/build/download_nacl_irt.py",
                "--nacl_revision", Var("nacl_revision"),
                "--file_hash", "x86_32", Var("nacl_irt_hash_x86_32"),
                "--file_hash", "x86_64", Var("nacl_irt_hash_x86_64")],
   },
 ]