Headless: expose WebPreferences to embedder, add --hide-scrollbars.

Lets the embedder override WebPreferences for individual HeadlessBrowserContexts. Used to add back the --hide-scrollbars flag to headless_shell.

BUG=546953

Review-Url: https://codereview.chromium.org/2374063002
Cr-Commit-Position: refs/heads/master@{#422074}
11 files changed