Slashes and backslashes are interchangeable on Windows in many contexts.
But not all. In particular, if you add a path to a dictionary and then
try looking it up with an equivalent path that has different slashes
then - not surprisingly - the lookup will fail. Therefore it is
important to always use slashes when populating _paths_to_owners. This
is now done, and documented.

Additionally, while fnmatch.fnmatch handles comparing slash separated
paths to backslash separated paths, fnmatch.translate does not. This is
arguably a bug in fnmatch.translate, or at least something that should
be documented. This bug is worked around by having _fnmatch sanitize
file names to always use slashes, and asserting that the patterns do
not contain backslashes.

With these changes this command:
  git cl owners --ignore-current --show-all chrome/browser/BUILD.gn
now correctly gives the same results as this command on Windows:
  git cl owners --ignore-current --show-all chrome\browser\BUILD.gn

The modified test fails without the other changes, passes with them.

Bug: 1009104
Change-Id: I416c7131281f00e352c1d2b85c30fcc463417fa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1915410
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>