GitLab

It's a goodness to sequester build configuration in the build/
directory. The gn secondary_source stuff was under tools/gn for
historical reasons, so move it to a more appropriate place.

BUG=None
TEST=do a gn build of base_unittests, run said unittests.

Review URL: https://codereview.chromium.org/340153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278355 0039d316-1c4b-4281-b951-d872f2087c98