[iOS] Replace deprecated MDCSnackbarManager.showMessage in remoting.

Deprecated 'showMessage:' causes Chrome internal BUILD failures.
Use [MDCSnackbarManager.defaultManager showMessage:] instead.

Bug: 1117399
Change-Id: Ib7e187d685b53c625bfed785d1753afb3d22762d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362248Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799239}

remoting/ios/session/remoting_client.mm

@@ -130,7 +130,7 @@ static void ResolveFeedbackDataCallback(
   info.capabilities = "";
   if ([RemotingPreferences.instance boolForFlag:RemotingFlagUseWebRTC]) {
     info.flags = "useWebrtc";
-    [MDCSnackbarManager
+    [MDCSnackbarManager.defaultManager
         showMessage:[MDCSnackbarMessage messageWithText:@"Using WebRTC"]];
   }