Fix inconsistency.

On linux, pkg-config.py outputs a 4-tuple. The non-linux fallback path
outputs only a 3-tuble, which confuses pkg_config.gni.

BUG=none
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/120973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242456 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242456