RecreateSKPs: Fix GoogleStorageDataStore.delete_path

BUG=skia:5843
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2423093002

Review-Url: https://codereview.chromium.org/2423093002
diff --git a/tools/skp/webpages_playback.py b/tools/skp/webpages_playback.py
index 1320a86..2e2b6a4 100644
--- a/tools/skp/webpages_playback.py
+++ b/tools/skp/webpages_playback.py
@@ -485,7 +485,7 @@
     return True
 
   def delete_path(self, path):
-    subprocess.check_call(['gsutil', 'rm', '-r', path])
+    subprocess.check_call(['gsutil', 'rm', '-r', '/'.join((self._url, path))])
 
   def download_file(self, name, local_path):
     subprocess.check_call([