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Rather than talking to X11, in headless mode, the native viewport service just
drops requests on the floor. This configuration is useful for writing
integration tests for the Mojo system that don't need to interact with X11.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/606883002

Cr-Commit-Position: refs/heads/master@{#296846}