Add a keep-alive mojo service for extensions.

This will allow apps and extensions APIs implemented on mojo services to
match the existing behavior of keeping background pages alive while API
calls are in progress.

BUG=389016

Review URL: https://codereview.chromium.org/673623004

Cr-Commit-Position: refs/heads/master@{#300853}
8 files changed