gn: don't reject absolute unix style pathnames in labels
Absolute paths have been suggested by brettw as a way for external products
based upon chromium to include code outside the chromium source tree. gn
supports absolute paths in theory, but this has not been tried in practice yet.
This patch is the first step towards testing this proposed setup.
Some background details:
https://groups.google.com/a/chromium.org/forum/#!topic/gn-dev/CU7MzRLrQuE
BUG=445454
R=brettw@chromium.org
Review URL: https://codereview.chromium.org/
825013003
Cr-Commit-Position: refs/heads/master@{#309753}