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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}