Without this repos that use the use_relative_paths still need to use
their absolute path for gclient_gn_args_file.

Bug: chromium:1102833
Change-Id: I470096625061a28abf495f4d1035121edbcd581f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2279874Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>