Fix owners.py all_possible_owners API

Previously this API didn't work unless some other API
had first been called to load the data.

Change-Id: Iff9b36c48b45b64dc2f9b95230955858501903dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2528111Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>

owners.py

@@ -574,6 +574,7 @@ class Database(object):
     subsequent math easier).
     """
 
+    self.load_data_needed_for(dirs_and_files)
     all_possible_owners_for_dir_or_file_cache = {}
     all_possible_owners = {}
     for current_dir in dirs_and_files: