V8: Let peek-gn use mb to reuse v8-side configs.

scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___predictable_bisect.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.v8",
       "--builder",
       "V8 Linux - predictable",
@@ skipping 123 matching lines @@
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/cleanup_temp.py"
     ],
     "name": "cleanup_temp"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
-      "gen",
-      "[TMP_BASE]/v8_gn_tmp_1",
-      "--args=is_debug=false target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "RECIPE_MODULE[build::v8]/resources/patch_mb_config.py",
+      "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
+      "/path/to/tmp/"
     ],
-    "name": "gn"
+    "name": "patch mb config (fyi)"
   },
   {
     "cmd": [
+      "python",
+      "-u",
+      "[SLAVE_BUILD]/v8/tools/mb/mb.py",
+      "gen",
+      "-m",
+      "client.v8",
+      "-b",
+      "V8 Linux - predictable",
+      "--config-file",
+      "[mb config]",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "[TMP_BASE]/v8_gn_tmp_1"
+    ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "generate_build_files with gn (fyi)"
+  },
+  {
+    "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
       "[TMP_BASE]/v8_gn_tmp_1",
       "[SLAVE_BUILD]/v8/out/Release",
       "all",
       "all"
     ],
     "name": "compare build flags (fyi)"
   },
@@ skipping 488 matching lines @@
     },
     "name": "Bisect a0.gclient runhooks",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
-      "gen",
-      "[TMP_BASE]/v8_gn_tmp_2",
-      "--args=is_debug=false target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "RECIPE_MODULE[build::v8]/resources/patch_mb_config.py",
+      "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
+      "/path/to/tmp/"
     ],
-    "name": "Bisect a0.gn",
+    "name": "Bisect a0.patch mb config (fyi)",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
+      "[SLAVE_BUILD]/v8/tools/mb/mb.py",
+      "gen",
+      "-m",
+      "client.v8",
+      "-b",
+      "V8 Linux - predictable",
+      "--config-file",
+      "[mb config]",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "[TMP_BASE]/v8_gn_tmp_2"
+    ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "Bisect a0.generate_build_files with gn (fyi)",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
       "[TMP_BASE]/v8_gn_tmp_2",
       "[SLAVE_BUILD]/v8/out/Release",
       "all",
       "all"
@@ skipping 268 matching lines @@
     },
     "name": "Bisect a2.gclient runhooks",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
-      "gen",
-      "[TMP_BASE]/v8_gn_tmp_3",
-      "--args=is_debug=false target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "RECIPE_MODULE[build::v8]/resources/patch_mb_config.py",
+      "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
+      "/path/to/tmp/"
     ],
-    "name": "Bisect a2.gn",
+    "name": "Bisect a2.patch mb config (fyi)",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
+      "[SLAVE_BUILD]/v8/tools/mb/mb.py",
+      "gen",
+      "-m",
+      "client.v8",
+      "-b",
+      "V8 Linux - predictable",
+      "--config-file",
+      "[mb config]",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "[TMP_BASE]/v8_gn_tmp_3"
+    ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "Bisect a2.generate_build_files with gn (fyi)",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
       "[TMP_BASE]/v8_gn_tmp_3",
       "[SLAVE_BUILD]/v8/out/Release",
       "all",
       "all"
@@ skipping 303 matching lines @@
     },
     "name": "Bisect a1.gclient runhooks",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
-      "gen",
-      "[TMP_BASE]/v8_gn_tmp_4",
-      "--args=is_debug=false target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "RECIPE_MODULE[build::v8]/resources/patch_mb_config.py",
+      "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
+      "/path/to/tmp/"
     ],
-    "name": "Bisect a1.gn",
+    "name": "Bisect a1.patch mb config (fyi)",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
+      "[SLAVE_BUILD]/v8/tools/mb/mb.py",
+      "gen",
+      "-m",
+      "client.v8",
+      "-b",
+      "V8 Linux - predictable",
+      "--config-file",
+      "[mb config]",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "[TMP_BASE]/v8_gn_tmp_4"
+    ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "Bisect a1.generate_build_files with gn (fyi)",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
       "[TMP_BASE]/v8_gn_tmp_4",
       "[SLAVE_BUILD]/v8/out/Release",
       "all",
       "all"
@@ skipping 132 matching lines @@
     "~followup_annotations": [
       "@@@STEP_LINK@a2@https://chromium.googlesource.com/v8/v8/+/a2@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Failures or flakes in build.",
     "status_code": 1
   }
 ]