chromium_tests: use correct cwd for deapplying patch

scripts/slave/recipe_modules/chromium_tests/api.py

Index: scripts/slave/recipe_modules/chromium_tests/api.py
diff --git a/scripts/slave/recipe_modules/chromium_tests/api.py b/scripts/slave/recipe_modules/chromium_tests/api.py
index 5d5c101df3cbd5afc97625118cbcdc9b988beb6b..69884a3c73e42bae76e73e7c4a064e70def79cd2 100644
--- a/scripts/slave/recipe_modules/chromium_tests/api.py
+++ b/scripts/slave/recipe_modules/chromium_tests/api.py
@@ -644,8 +644,13 @@ class ChromiumTestsApi(recipe_api.RecipeApi):
     self.m.gclient.c.revisions[first_solution_name] = str(
         bot_update_json['properties'][rev_property])
     self._resolve_fixed_revisions(bot_update_json)
+
+    kwargs = {}
+    if self._working_dir:
+      kwargs['cwd'] = self._working_dir
+
     self.m.bot_update.ensure_checkout(
-        force=True, patch=False, update_presentation=False)
+        force=True, patch=False, update_presentation=False, **kwargs)
     self.m.chromium.runhooks(name='runhooks (without patch)')
 
   def run_tests_on_tryserver(self, bot_config, api, tests, bot_update_step,