[v8] define linux64 gc stress trybot in src side

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}

infra/mb/mb_config.pyl

@@ -208,6 +208,7 @@
           'release_simulate_arm64_pointer_compression',
       'v8_linux64_compile_rel_xg': 'release_x64_test_features_trybot',
       'v8_linux64_dbg_ng': 'debug_x64_trybot',
+      'v8_linux64_gc_stress_custom_snapshot_dbg_ng': 'debug_x64_trybot_custom',
       'v8_linux64_gcc_compile_dbg': 'debug_x64_gcc',
       'v8_linux64_header_includes_dbg': 'debug_x64_header_includes',
       'v8_linux64_fyi_rel_ng': 'release_x64_test_features_trybot',
@@ -490,6 +491,8 @@
       'debug_bot', 'x64', 'minimal_symbols'],
     'debug_x64_trybot': [
       'debug_trybot', 'x64'],
+    'debug_x64_trybot_custom': [
+      'debug_trybot', 'x64', 'v8_snapshot_custom'],
 
     # Debug configs for x86.
     'debug_x86': [

infra/testing/builders.pyl

@@ -257,6 +257,15 @@
       {'name': 'v8testing', 'variant': 'minor_mc', 'shards': 1},
     ],
   },
+  'v8_linux64_gc_stress_custom_snapshot_dbg_ng_triggered': {
+    'tests': [
+      {
+        'name': 'mjsunit',
+        'test_args': ['--gc-stress', '--no-harness'],
+        'shards': 3,
+      },
+    ],
+  },
   'v8_linux64_fyi_rel_ng_triggered': {
     'tests': [
       # Stress sampling.