[root_presenter] Deemphasize scary-looking disconnection message.

Deemphasize the log message. The view-disconnected event happens even
for normal flows, so it isn't a reliable indicator that something
actually went wrong.

SCN-1446 #comment

TEST=CQ

Change-Id: I832afa6bd2c24095ded60a873267d0d948fbba22
diff --git a/garnet/bin/ui/root_presenter/app.cc b/garnet/bin/ui/root_presenter/app.cc
index 3386c35..b36c8c208 100644
--- a/garnet/bin/ui/root_presenter/app.cc
+++ b/garnet/bin/ui/root_presenter/app.cc
@@ -265,8 +265,7 @@
           }
           FXL_DCHECK(idx != presentations_.size());
 
-          FXL_LOG(ERROR)
-              << "Root presenter: Content view terminated unexpectedly.";
+          FXL_LOG(INFO) << "Root presenter: Content view terminated.";
           ShutdownPresentation(idx);
           break;
         }