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An It2Me connection requires an email for domain policy check.
We should use the email of the helpee instead of the helper.

Summary of changes:
1. Modifies it2meHelpeeChannel to fetch its token and the user's credentials
   using remoting.identity.
2. Implement the background page as an object to initialize remoting.identity
   appropriately.
3. Makes the consentDialog argument optional in remoting.identity as user
   consent is suggested as opposed to required in the chrome identity API.

BUG=454897

Review URL: https://codereview.chromium.org/896003002

Cr-Commit-Position: refs/heads/master@{#314651}