chromium_tests: use correct cwd for analyze diff

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 62df7a50e98a91a909008e39317c7e5142bbfd63..fdf44df19f674988f5f7da4b4920a1b7ec059b12 100644
--- a/scripts/slave/recipe_modules/chromium_tests/api.py
+++ b/scripts/slave/recipe_modules/chromium_tests/api.py
@@ -677,12 +677,13 @@ class ChromiumTestsApi(recipe_api.RecipeApi):
     """
     patch_root = self.m.gclient.calculate_patch_root(
         self.m.properties.get('patch_project'))
-    affected_files = self.m.tryserver.get_files_affected_by_patch(patch_root)
-    for i, path in enumerate(affected_files):
+    cwd = self._working_dir.join(patch_root) if self._working_dir else None
+    files = self.m.tryserver.get_files_affected_by_patch(patch_root, cwd=cwd)
+    for i, path in enumerate(files):
       path = str(path)
       assert path.startswith(relative_to)
-      affected_files[i] = path[len(relative_to):]
-    return affected_files
+      files[i] = path[len(relative_to):]
+    return files
 
   def analyze(self, affected_files, test_targets, additional_compile_targets,
               config_file_name, mb_mastername=None, mb_buildername=None,