chrome.gcdPrivate allows app to choose pairing method.
Added startPairing with pairing method parameter.
Changed some naming in gcd_private.idl
Changed PrivetV3Session from delegates to Callback. This makes code smaller and more readable.
Committed:
https://crrev.com/b6f8838a4a34a4b173cc6aebbedba791c80a560b
Cr-Commit-Position: refs/heads/master@{#302565}