Updating It2Me to use the new ValidatingAuthenticator class

This change moves the connection validation logic for It2Me from the host
factory and into a callback in the It2Mehost class.

A subsequent change will move the confirmation dialog into this new method as
well but that is a larger change and I wanted to keep the initial change small.

BUG=617185

Review-Url: https://codereview.chromium.org/2271933002
Cr-Commit-Position: refs/heads/master@{#415774}
8 files changed