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

scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux64___gcov_coverage.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.v8",
       "--builder",
       "V8 Linux64 - gcov coverage",
@@ skipping 151 matching lines @@
       "--directory",
       "[SLAVE_BUILD]/v8/out/Release",
       "--zerocounters"
     ],
     "name": "lcov zero counters"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
-      "gen",
-      "[TMP_BASE]/v8_gn_tmp_1",
-      "--args=is_debug=false target_cpu=\"x64\" 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 Linux64 - gcov coverage",
+      "--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 148 matching lines @@
     "~followup_annotations": [
       "@@@STEP_LINK@report@https://storage.googleapis.com/chromium-v8/linux64_gcov_rel/f2cbea7e89a45f46819777bb78a1f5bd0241f64c/index.html@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]