Enable building headless_shell in MAC. This is an initial implementation with a hidden window, rather than "true" headless.

Note: screenshot functionality is not working. This is due to screenshots on mac being created from window bounds withing the screen, so DCHECK
fails when window is hidden. See https://cs.chromium.org/chromium/src/ui/snapshot/snapshot_mac.mm?rcl=5fa975c1785e2a47c96471691447846d1be8b51d&l=19 for actual code
BUG=687407

Review-Url: https://codereview.chromium.org/2712093002
Cr-Commit-Position: refs/heads/master@{#453147}
6 files changed