shelf: Remove uneeded create render cache.

It seems the opaque background has no children anymore, and repaints
afterwards, so I don't think a render cache is needed.

Test: none
Bug: 10158843
Change-Id: I5f99340f177d8a655f231d3565c972fee337f062
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868916Reviewed-by: Mitsuru Oshima (slow) <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708020}

ash/shelf/shelf_widget.cc

@@ -257,7 +257,6 @@ void ShelfWidget::DelegateView::UpdateOpaqueBackground() {
         shelf->SelectValueForShelfAlignment(0, radius, 0),
         shelf->SelectValueForShelfAlignment(0, 0, radius),
     });
-    opaque_background_.AddCacheRenderSurfaceRequest();
   }
   opaque_background_.SetBounds(opaque_background_bounds);
   UpdateDragHandle();