GTTF: Increase percentage of manual try jobs going to recipe-based trybots.

This goes from 10% to 25% .

Over Jan 9 we got 13 jobs per builder with no problems.

BUG=317931
R=joi@chromium.org

Review URL: https://codereview.chromium.org/133603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244160 0039d316-1c4b-4281-b951-d872f2087c98
Add BUILD.gn file to build libpng.

This adds the BUILD.gn file so GN can generate the ninja files for this
third_party project.

BUG=None
TEST=gn, ninja -C out/Default libpng
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/123613008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244161 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244161