Add a showcase entry for the content widget.

ios/showcase/widget/BUILD.gn

 # 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.
 
 source_set("widget") {
   sources = [
+    "sc_content_widget_coordinator.h",
+    "sc_content_widget_coordinator.mm",
     "sc_search_widget_coordinator.h",
     "sc_search_widget_coordinator.mm",
   ]
   deps = [
+    "//ios/chrome/content_widget_extension:content_widget",
     "//ios/chrome/search_widget_extension:search_widget",
     "//ios/showcase/common",
   ]
   libs = [ "UIKit.framework" ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }