This is a reland of aabb893a
Original change's description:
> fuchsia: Set up for 3-sided roll to convert Magenta->Zircon
> 
> 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/668751
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Commit-Queue: Scott Graham <scottmg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48047}

TBR=bbudge@chromium.org

Bug: chromium:765754, chromium:707030
Change-Id: Ib6e99ca418af527014622614d07d295b6110f9d5
Reviewed-on: https://chromium-review.googlesource.com/670944Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48067}