chromium.run_mb: set cwd to checkout

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 266 matching lines @@
       "-m",
       "chromium.webrtc",
       "-b",
       "Win Builder",
       "--config-file",
       "[BUILDER_CACHE]\\Win_Builder\\src\\tools\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "//out/Release"
     ],
+    "cwd": "[BUILDER_CACHE]\\Win_Builder\\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",
@@ skipping 143 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@Win10 Tester<br />Win7 Tester<br />Win8 Tester@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]