... and use them to complete the BigInt function.

Bug: v8:6791
Change-Id: Ia36db86b92d1a0cfcb783516e04d6c0e3750f194
Reviewed-on: https://chromium-review.googlesource.com/737643
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48993}