GitLab

- Added WebPresentationReceiver::RemoveConnection() API
- Added WebPresentationReceiver pointer to ReceiverConnectionProxy. WebPresentationReceiver::RemoveConnection() is invoked when receiver connection changes to 'closed'
- In PresentationDispatcher::OnReceiverConnectionAvailable, pass receiver_ pointer to ReceiverConnectionProxy

BUG=713227

Review-Url: https://codereview.chromium.org/2874483002
Cr-Commit-Position: refs/heads/master@{#473773}