Add non-member non-mutating methods for common gfx::Rect operations.

ui/app_list/search_result_view.cc

Index: ui/app_list/search_result_view.cc
diff --git a/ui/app_list/search_result_view.cc b/ui/app_list/search_result_view.cc
index 7e4559dce604dbb033f5ed6c47adafbe597df86d..185416c739c7c1b3b6c29b8db7667169bdf42bff 100644
--- a/ui/app_list/search_result_view.cc
+++ b/ui/app_list/search_result_view.cc
@@ -200,8 +200,7 @@ void SearchResultView::OnPaint(gfx::Canvas* canvas) {
     canvas->FillRect(content_rect, kHoverAndPushedColor);
   }
 
-  gfx::Rect border_bottom(rect);
-  border_bottom.Subtract(content_rect);
+  gfx::Rect border_bottom = gfx::Subtraction(rect, content_rect);
   canvas->FillRect(border_bottom,
                    selected ? kSelectedBorderColor : kBorderColor);