Add unit test and showcase support for search widget.

ios/chrome/search_widget_extension/search_widget_view_controller_unittest.mm

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "ios/chrome/search_widget_extension/search_widget_view_controller.h"
+
+#import "ios/chrome/search_widget_extension/search_widget_view_controller.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "testing/platform_test.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+using SearchWidgetViewControllerTest = PlatformTest;
+
+TEST_F(SearchWidgetViewControllerTest, Alloc) {
+  SearchWidgetViewController* controller =
+      [[SearchWidgetViewController alloc] init];
+  ASSERT_NE(controller, nil);
+}