Media Engagement: implement shell of the media engagement service.

This is created the KeyedService, the WebContentsObserver and the
Feature flag. This is only boilerplate.

BUG=715051

Review-Url: https://codereview.chromium.org/2840813002
Cr-Commit-Position: refs/heads/master@{#470362}
10 files changed