Add ToastManager class to ash/public/cpp

Renamed ToastManager from ash/system/toast to ToastManagerImpl.
Added ToastManager class to ash/public/cpp to access the implementation
from chrome/.

Bug: 934053
Change-Id: I913dfd7e426f958d15e4e22ffcae724f8ebb668b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1647706
Commit-Queue: Anastasiia Nikolaienko <anastasiian@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668373}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index d7d0fbb..ce93989 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -21,7 +21,7 @@
 #include "ash/shell.h"
 #include "ash/system/status_area_widget.h"
 #include "ash/system/status_area_widget_delegate.h"
-#include "ash/system/toast/toast_manager.h"
+#include "ash/system/toast/toast_manager_impl.h"
 #include "ash/system/tray/system_tray_notifier.h"
 #include "base/bind.h"
 #include "base/debug/alias.h"