Fuchsia changed their kernel name from Magenta to Zircon and all the
functions and defines along with it. In order to be able to roll the SDK
in Chromium, we first need to land with this define added in v8, so that
can roll in to Chromium, then roll the Fuchsia SDK with this magic
define set (CHROMIUM_ROLLING_MAGENTA_TO_ZIRCON), then actually update v8
to reference zx_ instead of mx_ and roll that again.

Chromium-side for reference: https://chromium-review.googlesource.com/c/chromium/src/+/669139

Bug: chromium:765754, chromium:707030
Change-Id: I4ed5027f455d2346f431e7c700e87693348d5b79
Reviewed-on: https://chromium-review.googlesource.com/668751Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48047}