GCS: extract tarfile if the content is tarfile

Bug: b/324418194
Change-Id: I207f5352a7d00d5bf7a7587dfa60f216adeef7ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5346335
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Joanna Wang <jojwang@chromium.org>

gclient.py

@@ -2630,8 +2630,9 @@ class GcsDependency(Dependency):
                                 calculated=calculated_sha256sum,
                             ))
 
-        with tarfile.open(output_file, 'r:*') as tar:
-            tar.extractall(path=output_dir)
+        if tarfile.is_tarfile(output_file):
+            with tarfile.open(output_file, 'r:*') as tar:
+                tar.extractall(path=output_dir)
         self.WriteFilenameHash(calculated_sha256sum, hash_file)
 
     #override