Fix mojo::system::ChannelManager::GetChannelInfo(), hence WillShutdownChannel().

Oops, GetChannelInfo() copied code (from ShutdownChannel()) that also
removed the ChannelInfo. WillShutdownChannel() (and GetChannelInfo())
shouldn't remove it, while ShutdownChannel() should continue to.

R=cmasone@chromium.org

Review URL: https://codereview.chromium.org/727003003