roll-dep.py helpfully trims both trailing '/' and trailing '\'
characters, however it then fails if there are any '\' characters
embedded in the path. We can make this script friendlier for Windows
users by replacing '\' with '/'.

This was manually tested with a deps-roll of src\native_client.

Change-Id: I8fbd3395364612c85db39b234ee7dcbda705d0b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2260997Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>