The first call (i.e. not a retry) to CastMediaSinkServiceImpl::OpenChannels takes a nullptr for |backoff_entry|.  However, |backoff_entry| is notified of the connection failure in OnChannelOpened, but not created if null until OnChannelErrorMayRetry (where the failure_count() is initialized to 0).

This can be seen in the test CastMediaSinkServiceImplTest.TestOpenChannelFails: the default retry count is used (which is currently 3) and the test asserts that the number of retry failures is 4.