Some of the owners.py code does not change file path to use the slash
separator, causing owners check to crash on Windows. This CL fixes it.

Change-Id: I6e06a235dbb563ea45691f19b03d826b23184b2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2626090Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mandy Chen <mandy.chen@microsoft.com>