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This CL adds an optional parameter to start_host which will be used to
verify that the auth_code provided was generated by the expected
user account.  This flag is optional as it is only needed for scenarios
where the host owner is not manually setting up the instance.

Change-Id: I6f48d008c9ea59e94532f8f84d6a7d5cf20cf68a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495877
Commit-Queue: Joe Downing <joedow@google.com>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820858}