Convert MimeRegistry to use mojo.

This also adds an optional task runner parameter to
ServiceRegistry::AddService() so mojo service factories can be run on
their preferred thread without needing to write a boilerplate forwarding
function to post to that thread.

BUG=577685

Review-Url: https://codereview.chromium.org/2024363002
Cr-Commit-Position: refs/heads/master@{#397959}
18 files changed