Mojo API for launching the public session

This CL includes:
1. Add mojo api for launching the public session.
2. Preload the webui for the post login screens.

Bug: 809635
Change-Id: I8b3e3877f250ea193ec6a49f90f6a0f275bafbaf
Reviewed-on: https://chromium-review.googlesource.com/957309
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545233}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index 8bdb35fd..203bcc6 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -327,6 +327,15 @@
   login_screen_client_->RemoveUser(account_id);
 }
 
+void LoginScreenController::LaunchPublicSession(
+    const AccountId& account_id,
+    const std::string& locale,
+    const std::string& input_method) {
+  if (!login_screen_client_)
+    return;
+  login_screen_client_->LaunchPublicSession(account_id, locale, input_method);
+}
+
 void LoginScreenController::AddLockScreenAppsFocusObserver(
     LockScreenAppsFocusObserver* observer) {
   lock_screen_apps_focus_observers_.AddObserver(observer);