Moving It2Me confirmation prompt into the Validation callback flow

This change moves the confirmation dialog prompt used in It2Me from
the beginning of the connection flow to the point where we know
the client JID.  This change allows the user to have more control
over who connects to their session (they can effectively 'see who
is calling' and reject it if it isn't an address they recognize).

BUG=645540

Review-Url: https://codereview.chromium.org/2310303002
Cr-Commit-Position: refs/heads/master@{#417960}
9 files changed