[owners] Implement ListOwnersForFile for Depot Tools

Add DepotToolsClient to implement the OwnersClient API
for Depot Tools, and implement the ListOwnersForFile method.

Change-Id: I933a262898439d879c919d695aa62d7702b4c9a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2530509
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

owners_client.py

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import owners
+
 
 class OwnersClient(object):
   """Interact with OWNERS files in a repository.
@@ -24,10 +26,21 @@ class OwnersClient(object):
     """List all owners for a file."""
     raise Exception('Not implemented')
 
-  def IsChangeApproved(self, change_number):
+  def IsChangeApproved(self, change_id):
     """Check if the latest patch set for a change has been approved."""
     raise Exception('Not implemented')
 
-  def IsOwnerConfigurationValid(self, change_number, patch):
+  def IsOwnerConfigurationValid(self, change_id, patch):
     """Check if the owners configuration in a change is valid."""
     raise Exception('Not implemented')
+
+
+class DepotToolsClient(OwnersClient):
+  """Implement OwnersClient using owners.py Database."""
+  def __init__(self, host, root):
+    super(DepotToolsClient, self).__init__(host)
+    self._root = root
+    self._db = owners.Database(root, open, os.path)
+
+  def ListOwnersForFile(self, _project, _branch, path):
+    return sorted(self._db.all_possible_owners([arg], None))