Adds the src side definitions for integrating
"V8 Linux64 GC Stress - custom snapshot" waterfall bot into the try jobs.

R=machenbach@chromium.org
CC=​yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8718
No-Try: true
Change-Id: Ie75846486b15127ea35ef6bbd478beb513f6d299
Reviewed-on: https://chromium-review.googlesource.com/c/1478194
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59739}