Add unit test and showcase support for search widget.

ios/showcase/core/showcase_model.mm

Index: ios/showcase/core/showcase_model.mm
diff --git a/ios/showcase/core/showcase_model.mm b/ios/showcase/core/showcase_model.mm
index ee7d3da1b8a7996d1b79afaf07bacc119d12326e..ded20c104143a4e63766272964d968c9fd69b651 100644
--- a/ios/showcase/core/showcase_model.mm
+++ b/ios/showcase/core/showcase_model.mm
@@ -81,6 +81,11 @@
       showcase::kClassForInstantiationKey : @"UIKitTableViewCellViewController",
       showcase::kUseCaseKey : @"UIKit Table Cells",
     },
+    @{
+      showcase::kClassForDisplayKey : @"SearchWidgetViewController",
+      showcase::kClassForInstantiationKey : @"SearchWidgetViewController",
+      showcase::kUseCaseKey : @"Search Widget",
+    },
   ];
 }