[iOS] Replace MDCSnackbarManager.showMessage in remoting_view_controller

TBR=yuweih@chromium.org

Bug: 1117399
Change-Id: I72152e4d14ff627837edfc3ba98d2b120ca9ba17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362615Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799632}

remoting/ios/app/remoting_view_controller.mm

@@ -381,7 +381,7 @@ animationControllerForDismissedController:(UIViewController*)dismissed {
   if ([self isAnyRefreshControlRefreshing]) {
     // User could just try pull-to-refresh again to refresh. We just need to
     // show the error as a toast.
-    [MDCSnackbarManager
+    [MDCSnackbarManager.defaultManager
         showMessage:[MDCSnackbarMessage messageWithText:errorText]];
     [self stopAllRefreshControls];
     return;