Ignore the shelf arrow button in spoken feedback

A shelf arrow button should not get the accessibility focus ring.
Because when spoken feedback is enabled, if a hidden shelf icon receives
the accessibility focus, scrollable shelf will ensure the icon to be
visible by scroll animation. So ignore the arrow button in
spoken feedback.

Bug: 1086943
Change-Id: Ibe82d0315bcd71b01255fe496ccba875f38e7fe3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218447
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772347}

ash/shelf/scrollable_shelf_view.cc

@@ -275,6 +275,12 @@ class ScrollableShelfView::ScrollableShelfArrowView
     SetEventTargeter(std::make_unique<views::ViewTargeter>(this));
     SetPaintToLayer();
     layer()->SetFillsBoundsOpaquely(false);
+
+    // When the spoken feedback is enabled, scrollable shelf should ensure that
+    // the hidden icon which receives the accessibility focus shows through
+    // scroll animation. So the arrow button is not useful for the spoken
+    // feedback users. The spoken feedback should ignore the arrow button.
+    GetViewAccessibility().OverrideIsIgnored(/*value=*/true);
   }
   ~ScrollableShelfArrowView() override = default;