Use gclient_utils.rmtree() when updating a GCS dep to handle readonly files on Windows.

Bug: 438515314
Change-Id: I84848b9589dbe761be0dc7d8427a1dd317d70da1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6851010
Auto-Submit: Brian Ryner <bryner@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

gclient.py

@@ -849,7 +849,7 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
                     # exist.
                     logging.warning(
                         'GCS dependency %s new version, removing old.', name)
-                    shutil.rmtree(gcs_deps[0].output_dir)
+                    gclient_utils.rmtree(gcs_deps[0].output_dir)
             else:
                 url = dep_value.get('url')
                 deps_to_add.append(