Also simplify usage of Object.prototype methods in JS natives: now
that they're added in bootstrapper, no need to import them.

Change-Id: I0db53dd5acaed3aa0a5b46f730b31baa376e282d
Reviewed-on: https://chromium-review.googlesource.com/478574
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44659}