GitLab

Summary of changes:
1. Implements a rich UI to prompt for the user's permission to
   accept remote assistance from a hangout participant
2. Move getUrlParameter() to base.js
3. Show an interactive auth prompt if the user doesn't have an
   OAuth2 token for starting the It2Me host in v2 app. For v1
   app, do an auth redirect.
4. Tear down the It2MeHelpee channel if the user decline the
   remote assistance.

BUG=451591

Review URL: https://codereview.chromium.org/888323002

Cr-Commit-Position: refs/heads/master@{#314405}