Cros: Fix shelf background in overview mode

Make the shelf background opaque when in overview mode, so it matches
the color with the in-app shelf.

Fixed: 1017427
Change-Id: Ic633c9a2df6d8967cc606ebf869559dfba2f719b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879708Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709710}

ash/shelf/shelf_background_animator.cc

@@ -215,10 +215,10 @@ SkColor ShelfBackgroundAnimator::GetBackgroundColor(
       shelf_target_color = ShelfConfig::Get()->GetShelfWithAppListColor();
       break;
     case SHELF_BACKGROUND_DEFAULT:
-    case SHELF_BACKGROUND_OVERVIEW:
       shelf_target_color = ShelfConfig::Get()->GetDefaultShelfColor();
       break;
     case SHELF_BACKGROUND_MAXIMIZED:
+    case SHELF_BACKGROUND_OVERVIEW:
       shelf_target_color = ShelfConfig::Get()->GetMaximizedShelfColor();
       break;
     case SHELF_BACKGROUND_OOBE: