With https://chromium-review.googlesource.com/2279874, gclient started
taking |use_relative_paths| into account when calculating the path to
gclient_args.gni from |gclient_gn_args_file|. This CL introduced an
issue where the calculation depends on the current working directory.
e.g. Running gclient from ~/repo and ~/repo/sub_dir produced different
results for gclient_args.gni.

Fix this by prepending the root path, which is absolute, when
calculating the path to gclient_args.gni.

Bug: chromium:1102833
Change-Id: I970e38352aea77d857e49dc7dca58f21ba8e0331
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2296219Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>