Hide back button after Gaia auth succeeds and 'please wait' is showing

Bug: 766964
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3ea67c04d2c1f825848ba015714a45661c0771bb
Reviewed-on: https://chromium-review.googlesource.com/724125Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509659}

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

@@ -1017,6 +1017,7 @@ login.createScreen('GaiaSigninScreen', 'gaia-signin', function() {
       }
 
       this.loading = true;
+      $('signin-back-button').hidden = true;
 
       // Now that we're in logged in state header should be hidden.
       Oobe.getInstance().headerHidden = true;