Add headless chrome as an accepted browser string in chromedriver.

This is working towards selenium support for headless see https://bugs.chromium.org/p/chromium/issues/detail?id=604324

BUG=699382,604324

Review-Url: https://codereview.chromium.org/2731403008
Cr-Commit-Position: refs/heads/master@{#456660}
2 files changed