Chromecast: adds disable_display define for headless builds.

Headless builds should include a few optimizations by default:
* Force-disabling GPU
* Not loading image resources

R=lcwu@chromium.org
BUG=336640

Review URL: https://codereview.chromium.org/837573003

Cr-Commit-Position: refs/heads/master@{#310546}
4 files changed