[es6] Use the correct ToPrimitive in the Date Constructor.

[es6] Use the correct ToPrimitive in the Date Constructor.

This way we can finally remove the ES5 ToPrimitive builtin from
runtime.js, and the Date Constructor now properly supports
@@toPrimitive for the single argument case as well.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux_layout_dbg,v8_linux_nosnap_dbg
R=rossberg@chromium.org
BUG=v8:4307
LOG=n

Review URL: https://codereview.chromium.org/1346893003

Cr-Commit-Position: refs/heads/master@{#30832}