Abstract equality comparison of a BigInt and a String converts the
latter to BigInt. This conversion can fail; since we do not want to
pass a context to the comparison function, we must signal such failure
without throwing an exception.
This CL uses the existing ShouldThrow enum to configure behavior of
String-to-BigInt conversion, moving it out of Object into globals.h.

Bug: v8:6791, v8:6979
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ibb98675079b8392cf03bbcbbbd5556108500a32d
Reviewed-on: https://chromium-review.googlesource.com/734172
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48946}