[iOS] Migrate MDCSnackbarManager dismiss call in remoting.

Migrates MDCSnackbarManager dismissAndCallCompletionBlocksWithCategory
to MDCSnackbarManager.defaultManager.

TBR=yuweih@chromium.org

Bug: 1117399
Change-Id: Iabfebf3086e2909a0dee1a4aedc47d8b19d2956e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363758
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799659}

remoting/ios/app/remoting_view_controller.mm

@@ -394,7 +394,8 @@ animationControllerForDismissedController:(UIViewController*)dismissed {
   // label.
   // TODO(yuweih): See if we really need to hide the account menu in this case,
   // since it requires nontrivial changes.
-  [MDCSnackbarManager dismissAndCallCompletionBlocksWithCategory:nil];
+  [MDCSnackbarManager.defaultManager
+      dismissAndCallCompletionBlocksWithCategory:nil];
 
   _fetchingErrorViewController.label.text = errorText;
   remoting::SetAccessibilityFocusElement(_fetchingErrorViewController.label);