Infer exact types for the various Date getter builtins, and also inline
the Date.prototype.getTime() builtin, which just returns the Date value
and thus doesn't need to check the cache stamp.

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2285213002
Cr-Commit-Position: refs/heads/master@{#38973}