If an OWNERS file used the file: directive with a relative file
path, but was using a root other than '/' (e.g.
'/path/to/my/real/root'), then the include resolver would incorrectly
leave a leading '/' on the include path. When os_path.join was then
called, the leading '/' meant the path was treated as an absolute path
and the join did not behave as expected.

Review-Url: https://codereview.chromium.org/2148683003