Switch 'Win x64' builder on chromium waterfall to use MB.

scripts/slave/recipes/chromium.expected/full_chromium_Win_x64.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 200 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]\\Win_x64\\src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json",
+      "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_DEFINES": "component=static_library fastbuild=1 gomadir='[CACHE]\\cipd\\goma' linux_dump_symbols=0 target_arch=x64 test_isolation_mode=noop use_goma=1",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILDER_CACHE]\\Win_x64\\src\\testing\\scripts\\get_compile_targets.py",
@@ skipping 36 matching lines @@
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\cleanup_temp.py"
     ],
     "name": "cleanup_temp"
   },
   {
     "cmd": [
       "python",
       "-u",
+      "[BUILDER_CACHE]\\Win_x64\\src\\tools\\mb\\mb.py",
+      "gen",
+      "-m",
+      "chromium",
+      "-b",
+      "Win x64",
+      "--config-file",
+      "[BUILDER_CACHE]\\Win_x64\\src\\tools\\mb\\mb_config.pyl",
+      "--goma-dir",
+      "[CACHE]\\cipd\\goma",
+      "//out/Release_x64"
+    ],
+    "cwd": "[BUILDER_CACHE]\\Win_x64\\src",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
+    "name": "generate_build_files"
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\tools\\runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\compile.py",
       "--gsutil-py-path",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gsutil.py",
       "--ninja-path",
       "RECIPE_PACKAGE_REPO[depot_tools]\\ninja.exe",
       "--target",
       "Release_x64",
@@ skipping 47 matching lines @@
       "dev"
     ],
     "name": "archive_build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]