The build config inherited from Chromium only enables a subset
of the checks that UBSan supports. We want them all, so this
patch overrides what "is_ubsan" means for V8.

Bug: v8:3770
Change-Id: I1d0a7d994279272f13ff1d4ac9ed235fcbfc0951
Reviewed-on: https://chromium-review.googlesource.com/c/1443502
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59222}