[Lockscreen Media Controls] Fix controls not showing after youtube ads.

Bug: 1015547
Change-Id: Ieade9352ce689c07f22710e11545adf571d93446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867193Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709746}

ash/login/ui/lock_screen_media_controls_view.cc

@@ -743,7 +743,9 @@ void LockScreenMediaControlsView::HideArtwork() {
 }
 
 void LockScreenMediaControlsView::SetShown(Shown shown) {
-  DCHECK(!shown_);
+  if (shown_ == shown)
+    return;
+
   shown_ = shown;
 
   base::UmaHistogramEnumeration(kMediaControlsShownHistogramName, shown);