[chromedriver] Introduce chromeOptions.useAutomationExtension.

This allows clients to enable or disable the ChromeDriver automation extension.
It is currently true by default, but once we introduce DevTools commands to
replace the extension, we'll set this to false by default, and later remove it.

BUG=604324
TBR=stgao@chromium.org

Review-Url: https://codereview.chromium.org/2785413002
Cr-Commit-Position: refs/heads/master@{#461032}
4 files changed