This is a reland of 7a0d6a50

Original change's description:
> [build] Disable slow asserts for v8_enable_fast_mksnapshot
>
> Slow asserts double the time spent in mksnapshot. While we want them
> enabled on our bot builds, local builds should be fast and don't
> necessarily need them.
>
> This also adds v8_enable_fast_mksnapshot as default in gm.py.
>
> Bug: v8:7891,v8:6688
> Change-Id: I5ab1491f3396c4351de30dbbb80a81e2242c5db9
> Reviewed-on: https://chromium-review.googlesource.com/1116552
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54074}

TBR=jkummerow@chromium.org

Bug: v8:7891, v8:6688
Change-Id: I3b1e80b3ca156b7d571ebc7f04e341f0ba716691
Reviewed-on: https://chromium-review.googlesource.com/1118139Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54080}