Adding <keygen> Content Setting (Android UI)

This adds Android UI to allow the user to change the Content Setting to block/allow keygen usage on a per-site basis.

To trigger the exception, we also create a Content Exception on any site that uses a <keygen> element.

BUG=514767

Review URL: https://codereview.chromium.org/1432573002

Cr-Commit-Position: refs/heads/master@{#370416}
11 files changed