Recognize Date.now() calls in the Typer and assign the proper integer
type to them.

See Node issue https://github.com/nodejs/node/issues/9729 for more
information.

R=yangguo@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2528853003
Cr-Commit-Position: refs/heads/master@{#41242}