unified-heap: Switch embedder snapshot type via GN flag

Instead of relying on OFFICIAL_BUILD, rely on a specific GN flag that can be
toggled.

V8 CL: http://crrev.com/c/1505372

Bug: 936797
Change-Id: I4e02e10be3b914c1c75f84b4d01d9d0b1b1d5ca1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504573
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639432}
5 files changed