GN: Return array from pkgconfig wrapper

This changes the pkg-config wrapper to support filtering out values (we'll need
this for some NSS stuff which will be in a followup patch) and parsing and
returning the results in a GN array in one go, rather than requiring two
invocations.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/24657003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225274 0039d316-1c4b-4281-b951-d872f2087c98