Oobe: disable shelf buttons when Gaia modal dialog opened

Bug: 1046710
Change-Id: I20db7a5eadf9c3503802991b87216f7f85209d92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2747597
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Yunke Zhou <yunkez@google.com>
Cr-Commit-Position: refs/heads/master@{#861544}

chrome/browser/resources/chromeos/login/gaia_dialog.js

@@ -212,9 +212,11 @@ Polymer({
       },
       'dialogShown': (e) => {
         this.navigationEnabled = false;
+        chrome.send('enableShelfButtons', [false]);
       },
       'dialogHidden': (e) => {
         this.navigationEnabled = true;
+        chrome.send('enableShelfButtons', [true]);
       },
       'exit': (e) => {
         this.fire('exit', e.detail);