This patch provides "Abstract Comparison" functions on the BigInt
class for comparing BigInts to Numbers and Strings.
The functionality is not exposed to JavaScript yet.

Bug: v8:6791
Change-Id: I835f290203a31f363970b1edb359e19af6dabc5d
Reviewed-on: https://chromium-review.googlesource.com/722324
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48718}