Includes drive-by fix of a small BigInt bug, as caught by
test262/built-ins/BigInt/constructor-from-string-syntax-errors

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ic3b78310912f84bbf904a1fcb7ddf2d7eb2df013
Reviewed-on: https://chromium-review.googlesource.com/817775Reviewed-by: Sathya Gunasekaran (ooo until 12/12) <gsathya@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50010}