cros: Hide scroll flash for Gaia on the login screen

Fixed: 1024239
Change-Id: I7685dcf0177213fa27262759b2c6af961d6d13d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2002869Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733315}

chrome/browser/chromeos/login/test/active_directory_login_mixin.cc

@@ -102,13 +102,15 @@ void ActiveDirectoryLoginMixin::ClosePasswordChangeScreen() {
 void ActiveDirectoryLoginMixin::TestLoginVisible() {
   OobeScreenWaiter screen_waiter(GaiaView::kScreenId);
   screen_waiter.Wait();
-  // Checks if Gaia signin is hidden.
-  test::OobeJS().ExpectHiddenPath({kGaiaSigninId, kGaiaSigninDialogId});
 
-  // Checks if Active Directory signin is visible.
+  // Wait for the Active Directory signin visible.
   std::initializer_list<base::StringPiece> ad_screen{kGaiaSigninId,
                                                      kAdOfflineAuthId};
-  test::OobeJS().ExpectVisiblePath(ad_screen);
+  test::OobeJS().CreateVisibilityWaiter(true, ad_screen)->Wait();
+
+  // Checks if Gaia signin is hidden.
+  test::OobeJS().ExpectHiddenPath({kGaiaSigninId, kGaiaSigninDialogId});
+
   test::OobeJS().ExpectNE(test::GetOobeElementPath(ad_screen) + ".clientWidth",
                           0);
   test::OobeJS().ExpectNE(test::GetOobeElementPath(ad_screen) + ".clientHeight",

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

@@ -945,7 +945,10 @@ Polymer({
     this.showViewProcessed_ = false;
     this.startLoadAnimationGuardTimer_();
     this.clearLoadingTimer_();
-    this.loadingFrameContents_ = false;
+    // Workaround to hide flashing scroll bar.
+    this.async(function() {
+      this.loadingFrameContents_ = false;
+    }.bind(this), 100);
 
     if (!this.$['offline-gaia'].hidden)
       this.$['offline-gaia'].focus();