Replace GetElement and SetElement to Get and Set, and use CopyElements,
which reduces 4x-13x overheads.

Bug: chromium:768775
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I58534b30c2035195c5f4b8f2c04e7c459bdbebaa
Reviewed-on: https://chromium-review.googlesource.com/720661Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48723}