Add GN build for parts of gpu and ozone.

Moves existing ozone platform flag to new .gni file with all ozone flags in it.

R=piman@chromium.org, rjkroege@chromium.org, spang@chromium.org

Review URL: https://codereview.chromium.org/285103002

git-svn-id: http://src.chromium.org/svn/trunk/src/third_party/khronos@270780 4ff67af0-8c30-449e-8e8b-ad334ec8d88c