Public API:
- Added incognito_mode flags to HeadlessBrowser::Options and
  HeadlessBrowserContext::Builder.

Headless Shell:
- Uses incognito browser context by default, switches to normal one
  when --user-data-dir flag is present.

BUG=546953

Review-Url: https://codereview.chromium.org/2211993002
Cr-Commit-Position: refs/heads/master@{#409847}