Full Firefox on Linux can now be run with a --headless flag.
This includes seven parts:
1) Running all marionette tests in headless mode.
2) Prevents crashes where Firefox calls into GTK.
3) Adds a headless screen helper which supports changing the headless
screen size with the environment variables MOZ_HEADLESS_WIDTH and
MOZ_HEADLESS_HEIGHT.
4) Supports simulating moving a headless window.
5) Adds a stubbed out nsSound implementation.
6) Supports simulating size mode changes of headless windows.
7) Adds the --headless flag for Firefox.
