gn finds // by searching .gn in parent directories.
To support out dir is not 2 directories from root (i.e. out/Default),
search .gn in parent directory for '//' location.
tools/licenses/licenses.py runs 'gn gen' in out/Default/$tmp.
for such case, '//' should be '../../..', or fail to import '//path'
Change-Id: I6e5ccbe93cb5e51704f31d4eb558c03560286865
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6051636
Reviewed-by: Philipp Wollermann <philwo@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>