Add a message to empty ContentSuggestions sections

ios/chrome/browser/ui/content_suggestions/BUILD.gn

 # Copyright 2016 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("content_suggestions") {
   sources = [
     "content_suggestion.h",
     "content_suggestion.mm",
     "content_suggestions_collection_updater.h",
     "content_suggestions_collection_updater.mm",
     "content_suggestions_commands.h",
     "content_suggestions_data_sink.h",
     "content_suggestions_data_source.h",
     "content_suggestions_image_fetcher.h",
     "content_suggestions_view_controller.h",
     "content_suggestions_view_controller.mm",
   ]
   deps = [
     "//base",
+    "//components/strings",
     "//ios/chrome/browser/ui/collection_view",
     "//ios/chrome/browser/ui/content_suggestions/cells",
     "//ios/chrome/browser/ui/content_suggestions/identifier",
     "//ui/base",
     "//url",
   ]
   public_deps = [
     "//ios/third_party/material_components_ios",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "content_suggestions_collection_updater_unittest.mm",
+  ]
+  deps = [
+    ":content_suggestions",
+    "//base",
+    "//ios/chrome/browser/ui/collection_view",
+    "//ios/chrome/browser/ui/content_suggestions/identifier",
+    "//testing/gtest",
+    "//third_party/ocmock",
+  ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}