Cloning arrays with Array.prototype.map and for-of push is too slow
for large arrays.

Bug: chromium:878681
Change-Id: I70ba5faad9e19cdc5a39cc64fb1b4bcb3fd0bf48
Reviewed-on: https://chromium-review.googlesource.com/1195363
Commit-Queue: Hai Dang <dhai@google.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55496}