Disable hotseat background blur when it is invisible

Bug: 1035943
Change-Id: Ic2cb63db33bd3bf3ccac8528fa1df53f5f3a3355
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976821Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Jun Liu <yjliu@chromium.org>
Auto-Submit: Jun Liu <yjliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726696}

ash/shelf/hotseat_widget.cc

@@ -165,6 +165,8 @@ void HotseatWidget::DelegateView::Init(
 void HotseatWidget::DelegateView::UpdateOpaqueBackground() {
   if (!ShouldShowHotseatBackground()) {
     opaque_background_.SetVisible(false);
+    if (features::IsBackgroundBlurEnabled())
+      opaque_background_.SetBackgroundBlur(0);
     return;
   }