Disable multi_snapshots by default

The multi-snapshot feature is only used on bots, where we need the
ability to execute tests with/without mitigations controllable through a
runtime flag.

This disables the flag by default and only enables it on bot configs.

Note: standard single-snapshot builds silently use the available
snapshot regardless of the value of --untrusted-code-mitigations.

Bug: v8:7891,v8:6688
Change-Id: I0d1a91e5c1c17b4d91b42d176b50ea44d2128d48
Reviewed-on: https://chromium-review.googlesource.com/1117687
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54079}

gni/v8.gni

@@ -38,7 +38,7 @@ declare_args() {
   v8_use_snapshot = !(is_win && host_os != "win" && target_cpu == "x64")
 
   # Enable several snapshots side-by-side (e.g. default and for trusted code).
-  v8_use_multi_snapshots = ""
+  v8_use_multi_snapshots = false
 
   # Use external files for startup data blobs:
   # the JS builtins sources and the start snapshot.
@@ -61,7 +61,10 @@ if (v8_use_external_startup_data == "") {
   v8_use_external_startup_data = v8_use_snapshot && !is_ios
 }
 
-if (v8_use_multi_snapshots == "") {
+if (v8_use_multi_snapshots) {
+  # Silently disable multi snapshots if they're incompatible with the current
+  # build configuration. This allows us to set v8_use_multi_snapshots=true on
+  # all bots, and e.g. no-snapshot bots will automatically do the right thing.
   v8_use_multi_snapshots =
       v8_use_external_startup_data && !build_with_chromium && !use_jumbo_build
 }

infra/mb/mb_config.pyl

@@ -590,10 +590,14 @@
       'gn_args': 'is_debug=true v8_enable_backtrace=true',
     },
 
+    'v8_use_multi_snapshots': {
+      'gn_args': 'v8_use_multi_snapshots=true',
+    },
+
     'debug_bot': {
       'mixins': [
         'debug', 'shared', 'goma', 'v8_enable_slow_dchecks',
-        'v8_optimized_debug'],
+        'v8_use_multi_snapshots', 'v8_optimized_debug'],
     },
 
     'debug_trybot': {
@@ -680,7 +684,7 @@
     },
 
     'release_bot': {
-      'mixins': ['release', 'static', 'goma'],
+      'mixins': ['release', 'static', 'goma', 'v8_use_multi_snapshots'],
     },
 
     'release_trybot': {