[CRD iOS] Fix "report this" when connection dropped

We release the reference to _client when ClientConnectionViewController
opens up the HostViewController. When the connection is dropped, the
ClientConnectionViewController pops out without transferring the
ownership of _client to ClientConnectionVC, so user now cannot send
feedback by tapping "report this".

There is no retain cycle between RemotingClient and ClientConnectionVC
or HostVC so it is fine to make ClientConnectionVC keep the reference
to _client after the HostVC is showing. Verified that no RemotingClient
persists in any scenarios after the connection ends.

Bug: 888277
Change-Id: I25940441327f5bf1a77eeb072d6fc45fc991e02f
Reviewed-on: https://chromium-review.googlesource.com/c/1287438Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600963}

remoting/ios/app/client_connection_view_controller.mm

@@ -484,7 +484,6 @@ using EntryPoint = remoting::ChromotingEvent::SessionEntryPoint;
 
   HostViewController* hostViewController =
       [[HostViewController alloc] initWithClient:_client];
-  _client = nil;
 
   [self.navigationController pushViewController:hostViewController animated:NO];
 }