[CrOS] Add install cloudready shelf button

Add Install CloudReady shelf button that will start the OS install
flow.
The button should only be visible on primary login screen if no
other dialogs are shown.

Bug: b:190523791
Change-Id: I3dbdfc9013ff01bf69897c1711055813273c1229
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2982698
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ossama Mahmoud <osamafathy@google.com>
Cr-Commit-Position: refs/heads/master@{#897358}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index cb4e711..dd35ffd 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -241,6 +241,12 @@
   client_->ShowGaiaSignin(prefilled_account);
 }
 
+void LoginScreenController::ShowOsInstallScreen() {
+  if (!client_)
+    return;
+  client_->ShowOsInstallScreen();
+}
+
 void LoginScreenController::OnRemoveUserWarningShown() {
   if (!client_)
     return;