app_list: Make corner radius same as search page

This patch matches the corner radius of assistant page and the search
result page. Also, since we have a mask layer, remove the background's
corner radius.

Bug: 924624
Test: manual
Change-Id: I640d1cd1d52495a9cd2ed405c16eaecc9cb9781c
Reviewed-on: https://chromium-review.googlesource.com/c/1496297Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636877}

ash/app_list/views/assistant/assistant_page_view.cc

@@ -40,13 +40,11 @@ void AssistantPageView::InitLayout() {
   SetPaintToLayer();
   layer()->SetFillsBoundsOpaquely(false);
 
-  SetBackground(views::CreateBackgroundFromPainter(
-      views::Painter::CreateSolidRoundRectPainter(
-          SK_ColorWHITE, search_box::kSearchBoxBorderCornerRadius)));
+  SetBackground(views::CreateSolidBackground(SK_ColorWHITE));
 
   mask_ = views::Painter::CreatePaintedLayer(
       views::Painter::CreateSolidRoundRectPainter(
-          SK_ColorBLACK, search_box::kSearchBoxBorderCornerRadius));
+          SK_ColorBLACK, search_box::kSearchBoxBorderCornerRadiusSearchResult));
   mask_->layer()->SetFillsBoundsOpaquely(false);
   layer()->SetMaskLayer(mask_->layer());