Remove duplicate set bounds for wallpaper

The window containers for wallpaper all have fill layout, there is no
need to set the bounds manually.

Bug: None
Test: manual and unit test
Change-Id: I60cd9940ec64ab4afe924377cabe8dd7b6b6e908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406670Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806711}

ash/wallpaper/wallpaper_view.cc

@@ -256,9 +256,6 @@ std::unique_ptr<views::Widget> CreateWallpaperWidget(
                                                  ::wm::ANIMATE_NONE);
   }
 
-  aura::Window* container = root_window->GetChildById(container_id);
-  wallpaper_widget->SetBounds(container->bounds());
-
   return wallpaper_widget;
 }